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Car Forum / Driving, Maintenance, Tuning / General Car Topics / October 2005Why do cars with automatic transmissions have tachometers?
Curious as hell about this. Was taught by the parents to drive in general on a 65 Chevy with an auto tranny. Then the parents taught me to drive a stick on a 67 Dodge 3-on-the-tree Dart. None of them had a tach. I presently drive a 5-speed sedan with a tach but I never notice it as I shift based on what my ear (and obviously "experience") tells me. Obviously I understand what the tachometer is for but.....my friends and relatives various vehicles are universally automatics and almost all - depending of course on the options they took - have tachometers on them. What's the point of the tachometer on a vehicle with an automatic transmission? Seriously, based on my understanding of WHY the tachometer exists, why are there tachs on vehicles with automatic transmissions? tach's are in cars today for a very simple reason. they make the idiots who buy and drive them feel "sporty". it has nothing to do with finding the shift points.a honda civic is a honda civic. but put a tach in it and a big wing on the back, and it is a CIVIC!!! see? it gots a tach . now i can go racing cause i gots a race car!!! its got to be one, cause its got a tach in it!! > Curious as hell about this. > [quoted text clipped - 19 lines] > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? (f.cking idiot text snipped) I drive in Montana. It's nice to know when I'm approaching redline on the tach when I have my foot to the floor. >(f.cking idiot text snipped) > >I drive in Montana. It's nice to know when I'm approaching redline on the >tach when I have my foot to the floor. Modern cars have rev limiters. ...NEXT!!!  Signature-john wide-open at throttle dot info > Curious as hell about this. > [quoted text clipped - 19 lines] > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? A tachometer on an automatic transmission vehicle can be very useful, especially for one that is electronically controlled. If I want to get the transmission to shift smoothly at something like 3000 rpm in each gear, I can gradually accelerate and then let off the throttle just a bit as the tachmeter nearly reaches 3000 rpm, at which time, the transmission will shift at the desired RPM's. Same goes for if I want the transmission to shift at 2500 RPM's or less (although that's really slow acceleration generally only used in residential areas) or at 4000 RPM's or more (climbing a bit of a grade). The accurate shifts at a given RPM can continue to occur until the transmission reaches its final gear and torque converter lockup subseqently occurs, thanks to the visual feedback of the tachometer. After a while, I've learned the acceleration rates for shifting at a given RPM, so I can maintain the throttle in a constant position such that the transmission shifts smoothly at the desired RPM. Without a tachometer, I could judge by engine sound, but not know the exact RPM's. And, it's also helpful to know when the engine is approaching redline under more spritied acceleration (e.g., flooring it on a very short interstate onramp that merges rather than joins with an extra lane, giving very little distance to get up to the 65mph freeway speed) rather than just waiting for the transmission to automatically shift to the next gear when the engine hits redline. Since my car's specifications indicate a maximum of 126 horsepower at 5500 RPM, there's really no need to exceed 5500 RPM, but I wouldn't know if I reached or exceeded that max engine torque RPM under higher acceleration without a tachometer. Others may vary in their usage or requirements for a tachometer. > > Curious as hell about this. > > [quoted text clipped - 53 lines] > > Others may vary in their usage or requirements for a tachometer. Same here. I use mine all the time for forcing up-shifts and as a general check on engine performance. Example: Hmmm...last month I was climbing this hill in 6th (od), what happened that I am now in 5th?. I can't feel or hear the diffeence in the 6/5 or 5/6 shift so see of the pants is no indicator. Harry K As Tom said, to make you think you got a sporty car. Useless as an udder on a turtle for most drivers. > As Tom said, to make you think you got a sporty car. > Useless as an udder on a turtle for most drivers. I found it useful to diagnose a TCC problem in my X-car, I could easily see what RPMs the clutch locked up and unlocked, and when I noticed that it wasn't locking and unlocking at the proper RPMs for the conditions I knew something was up. Sure enough, a few thousand miles later it failed, but thanks to the tach I had plenty of warning and was prepared to immediately repair the problem. JazzMan Signature********************************************************** Please reply to jsavage"at"airmail.net. Curse those darned bulk e-mailers! ********************************************************** "Rats and roaches live by competition under the laws of supply and demand. It is the privilege of human beings to live under the laws of justice and mercy." - Wendell Berry ********************************************************** > As Tom said, to make you think you got a sporty car. > Useless as an udder on a turtle for most drivers. Unless you want to know when you're getting ready to redline when the foot is to the floor. > Unless you want to know when you're getting ready to redline when the foot > is to the floor. We read what you had to say the first two times, idiot. > We read what you had to say the first two times, <idiot. At least I don't worship some child molesting prophet named Mohammed >> We read what you had to say the first two times, <idiot. > > At least I don't worship some child molesting prophet named Mohammed What's the name of your child molesting prophet you worship? Alan Johnson? > As Tom said, to make you think you got a sporty car. > Useless as an udder on a turtle for most drivers. But that's true for the manual transmissions as well. ;) > Curious as hell about this. > [quoted text clipped - 19 lines] > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? ive found factory tach's handy in diag. some questionable tranny shift probs.without having to hook up the scanner...shift points, shift slip, etc....i think your first RE; had it pegged, just another 'do-dad' like auto vol. control and steering wheel radio controls....as much as they say there cars are values they sure spend alot of time and money on crap thats not needed.........................kjun > What's the point of the tachometer on a vehicle with an automatic > transmission? To use current-day terminology: it's "bling". Signatureremove MYSHOES to email >>What's the point of the tachometer on a vehicle with an automatic >>transmission? > > To use current-day terminology: it's "bling". Probably also a matter of economy - cheaper to put them in all models rather than just the manual xmissions - what would fill the hole in the dash? > Curious as hell about this. Because it's nice to know when your getting ready to "redline" when you have your foot all the way to the floor. >>Curious as hell about this. > > Because it's nice to know when your getting ready to "redline" when you have > your foot all the way to the floor. Yeah, but an electronic auto trans will force a shift before "redline" no matter what you do ANYWAY. Hell- go out and get in many new cars (one I know for certain- Ford Expedition) and floor it in neutral and see what happens. OK, I'll tell you- the throttle-by-wire revs it up to about 3600 RPM and it sits there happy as a clam.. Some aren't quite as graceful- a GM 3800 I had as a rental didn't have throttle-by-wire, so it just cuts the fuel and the engine will "surge" around 4000 RPM. A tach would have been more useful in a 1970 car with an automatic than in a 2005 car with an automatic (not useful at ALL). Muscle cars frequently got tachs with automatics, because they do come in useful when manually shifting an auto. But back then, tachs went primarily in stick-shift cars where they REALLY belong. <Yeah, but an electronic auto trans will force a shift before "redline" <no matter what you do ANYWAY. Of course not. I was referring to redlining when approaching top end. Like at 130 mph in my IROC > <Yeah, but an electronic auto trans will force a shift before "redline" > <no matter what you do ANYWAY. > > Of course not. I was referring to redlining when approaching top end. Like > at 130 mph in my IROC An IROC (either Camaro or Daytona, you don't say which) is old enough that it might not have a rev limiter built in to the engine controller, and ghus goes along with my earlier comment that tachs on automatics are less useful *now* than they were years ago. Most modern cars, will still rev-limit at the top end as well as in neutral or in lower gears (and no, I'm not talking about the artificial top-speed limiter that many cars have- even performance cars without a speed limiter will still rev limit at redline now.) ><Yeah, but an electronic auto trans will force a shift before "redline" ><no matter what you do ANYWAY. > >Of course not. I was referring to redlining when approaching top end. Like >at 130 mph in my IROC I thought you were referring to my Enzo. Never mind. Signature-john wide-open at throttle dot info >Hell- go out and get in many new cars (one >I know for certain- Ford Expedition) and floor it in neutral and see >what happens. OK, I'll tell you- the throttle-by-wire revs it up to >about 3600 RPM and it sits there happy as a clam.. Some aren't quite as >graceful- a GM 3800 I had as a rental didn't have throttle-by-wire, so >it just cuts the fuel and the engine will "surge" around 4000 RPM. My old '87 T-Bird "redlines" at 5000, and the rev limit is factory set to 5500. I'm not kidding. BUT: I could buy a PROM chip which would crank it up to about 5700 RPM and modify the VE mapping, to give me about 9% more horsepower. Hmmm........ So, what is redline, anyway? 500 RPM before it blows up? :-) It's kind'a like the torque on Honda motorcycle cylinder head bolts. No torque wrench, or can't get the damn torque wrench in there? No problem! The correct torque is: "a quarter turn before it strips". Signature-john wide-open at throttle dot info > >Hell- go out and get in many new cars (one > >I know for certain- Ford Expedition) and floor it in neutral and see [quoted text clipped - 15 lines] > No torque wrench, or can't get the damn torque wrench in there? > No problem! The correct torque is: "a quarter turn before it strips". Isn't that usually quoted: "Torque it until it strips and then back off a quarter turn"? :-) SignatureAlan Baker Vancouver, British Columbia "If you raise the ceiling 4 feet, move the fireplace from that wall to that wall, you'll still only get the full stereophonic effect if you sit in the bottom of that cupboard." >> The correct torque is: "a quarter turn before it strips". > >Isn't that usually quoted: "Torque it until it strips and then back off >a quarter turn"? >:-) But by then, it's too late, you see... ...it's already been stripped! ;-> I'd hate to tell you how they test fuses... Signature-john wide-open at throttle dot info >>Curious as hell about this. > > Because it's nice to know when your getting ready to "redline" when you have > your foot all the way to the floor. Really? I hadn't heard that before. >Curious as hell about this. > [quoted text clipped - 19 lines] >Seriously, based on my understanding of WHY the tachometer exists, why >are there tachs on vehicles with automatic transmissions? It tends to help diagnose problems with one's transmission. -LMB > Curious as hell about this. Well, if you want to hold a low gear for acceleration up a hill or something, it's nice to know how hard you're thrashing the engine. Or setting your idle speed.... Tell you what, why do cars have oil pressure warning lights? I mean, you know if your car loses all oil pressure, right? The engine grinds to a halt and seizes... Why does your car have a temperature gauge? You'd know if it was overheating by the clouds of steam... >> Curious as hell about this. > [quoted text clipped - 8 lines] >Why does your car have a temperature gauge? You'd know if it was overheating >by the clouds of steam... It's getting hard to find cars with oil pressure gauges today, and even BMW is now promoting fake temperature gauges. Soon, even the idiot lights will all be replaced by one red light that says REPLACE ENGINE. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." Because it's nice to know when your getting ready to "redline" when you have your foot all the way to the floor. just had to say it too. :) > Curious as hell about this. > [quoted text clipped - 19 lines] > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? Becasue the car makers think people want them. I like having one even if I don't need one. Ed > Curious as hell about this. > [quoted text clipped - 5 lines] > > None of them had a tach. A better question would be, "Why do modern cars have useless instruments like a Tach, but lack useFUL instruments like oil pressure gauges calibrated in PSI, voltmeters calibrated in volts, or better yet ammeters." Oil temp and transmission temp gauges would be kinda nice too, especially when you're towing. The WORST of all are cars with a tach and NO other gauges, just idiot lights. FWIW- only one of my automatic cars (1993 Vison TSi) has a tach, and no I never "use" it. It is kinda nice to see when the torque convertor locks up to help save a little gasoline, but that's about it. Probably cheaper to produce one instrument cluster for everybody in models that can be bought with either an automatic or a stick, as is often the case. At the higher end of the market, it's also useful for the growing number of automatics that you can optionally run by hand. With LCD's getting cheaper and faster and brighter, I wonder when (at least in performance cars) they're going to start superimposing a virtual indicator on a display of power and torque curves, dyno-tested for at least the engine and driveline package if not the individual car. Cheers, --Joe Owner of two tach-less automatics, two tach-less manuals, and, until recently, one automatic with tach > Probably cheaper to produce one instrument cluster for everybody in > models that can be bought with either an automatic or a stick, as is [quoted text clipped - 13 lines] > Owner of two tach-less automatics, two tach-less manuals, and, until > recently, one automatic with tach The last time I looked (several years ago) it was rather difficult to find a manual option on most anything but the "sport" models. I'm waiting for them to do a fully configurable LCD based dashboard where you can load a "skin" for the instrument style and layout you prefer. Pretty simple to do and flexible enough that the same guts could be used on nearly every vehicle in the lineup, just changing the display bezel for the particular vehicle. Pete C. > I'm waiting for them to do a fully configurable LCD based dashboard > where you can load a "skin" for the instrument style and layout you > prefer. Pretty simple to do and flexible enough that the same guts could > be used on nearly every vehicle in the lineup, just changing the display > bezel for the particular vehicle. What is critical in that case is that they equip the enigne with calibrated sensors that can provide real data to the "skin" software. Its no good having a super-duper skin that shows oil pressure, amps, coolant temp, oil temp, and transmission temp if all you have is a voltage probe, an oil pressure idiot light switch, and a 3-state coolant temp idiot light switch :-/ > > I'm waiting for them to do a fully configurable LCD based dashboard > > where you can load a "skin" for the instrument style and layout you [quoted text clipped - 8 lines] > voltage probe, an oil pressure idiot light switch, and a 3-state coolant > temp idiot light switch :-/ In a lot of cases the computer has the real info, but they chose to just stick a cheap idiot light on the output. Pete C. > Probably cheaper to produce one instrument cluster for everybody in > models that can be bought with either an automatic or a stick, as is > often the case. But given that the bulk of models sold are ONLY available with an automatic..... its just bling-bling, as another poster already said. And frankly, you have to be a pretty incompetent driver to actually *use* the tach to decide when to shift on a routine basis. I always shift by engine sound, although I will check the tach a time or two the first time I drive a stick car that I'm not accustomed to (insert obligatory story about first-time RX-7 driving experience here...). Even drag racers don't use the tach- they generally use a shift-light triggered by a digital tach, not the tach itself. > Curious as hell about this. > [quoted text clipped - 19 lines] > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? Well, my feelings on this subject are mixed. Your Otto Cycle engine in your auto-tranny equipped vehicle will be most fuel-efficient in the powerband range of certain RPMs. SO, if you know how to find that fuel-efficient point*, you can try to run your engine at a certain RPM to maximize fuel economy. Obviously, if you don't have a tachometer, you can't do this. But then again, for some vehicles, it doesn't matter whether there is a tachometer or not. Take my wife's car for example. I ran the numbers to determine where it would be most fuel-efficient. It should be most fuel-efficient from 3200-3600RPM. Problem is, I discovered that 75MPH was somewhere below 3000RPM with overdrive enabled. So to get to 3200 minimum would be ummmmm, not a good idea. With overdrive OFF, RPM jumped up above 4000. Basically, I discovered that my wife's car was geared incorrectly. So in the case of that specific vehicle, a tachometer is useless. I can't drive it legally AND keep the engine in the powerband. -Dave * Various sources say 40% of redline, 40-45% of redline, or 60% of RPM at maximum horsepower RPM. In the case of my wife's car, this means it will be most fuel-efficient in a range from 3200-3600RPM. > Well, my feelings on this subject are mixed. Your Otto Cycle engine in your > auto-tranny equipped vehicle will be most fuel-efficient in the powerband > range of certain RPMs. > * Various sources say 40% of redline, 40-45% of redline, or 60% of RPM at > maximum horsepower RPM. In the case of my wife's car, this means it will be > most fuel-efficient in a range from 3200-3600RPM. The implicit assumption in that sort of "rule of thumb" is that you NEED all the power that the engine can produce. Its a good rule for, say, an industrial generator engine or irrigation well engine that is expected to run most of its life at maximum load Cars cruising down the highway NEVER need all the power that the engine can produce, so they play by a different set of rules. Cars have engines that are capable of producing several hundred horsepower in order to achieve good acceleration, but when cruising steady-state they require a few tens of horsepower at most. So how do you run a 300-horsepower engine at an output of 35 horsepower and do it most efficiently? It turns out that the best way is to "lug" the engine. IOW, run it so that its way down on the lower RPM side of its torque curve, but do so with the throttle held relatively wide open at that low RPM. that's why overdrive works so well. Lugging the engine does a few things simultaneously: 1) it raises the starting pressure in the cylinders by decreasing the amount of manifold vacuum. Higher pre-combustion pressure -> more efficient combustion 2) it allows the spark timing to be retarded. Spark "advance" is really only needed because when you operate an engine against very high manifold vacuum (throttle mostly closed) you are LOWERING the starting cylinder pressure so much that the burn rate is reduced, so that in turn you need to start the burn very early in order to finish it before the exhaust valve opens. The downside to spark advance is that a portion of the combustion cycle is working against the movement of the piston. If you lug the engine with the throttle open, the pressure is high so the burn rate is high, and you don't need much spark advance, and therefore less of the combustion cycle is spent working against the crankshaft. The proof is in the pudding. If you look at that (otherwise useless :-) ) tach in any modern car, you'll see that when cruising in O/D at 70 mph, the engine is only turning 2000 RPM or even less. And this is usually with engines that have redlines of 5000-7000 RPM and torque peaks up at 3000-4000 RPM. By any rule of thumb relating to their MAXUMUM power, the engines are being operated way, way off optimum. But for the NEEDED power, they're right in the sweet spot. > The implicit assumption in that sort of "rule of thumb" is that you NEED > all the power that the engine can produce. Its a good rule for, say, an > industrial generator engine or irrigation well engine that is expected > to run most of its life at maximum load Cars cruising down the highway > NEVER need all the power that the engine can produce, so they play by a > different set of rules. Ummmmm, no. The most fuel-efficient RPM of an Otto Cycle engine is well below the RPM that will produce the most horsepower. "Lugging" the engine is not fuel-efficient, as the engine RPM is too low. At low RPM, too much energy is wasted as heat. (a lesser percentage of energy is converted to kinetic energy while the engine is lugging) Overdrive is meant to improve fuel efficiency at higher speeds. (OVER drive speeds) It can't do this by "lugging" the engine. It does this by reducing the RPM into a more fuel-efficient range while still NOT lugging it. Good example: My car is at 3500RPM/43%(redline) and .6 (of max horsepower RPM) at 78MPH in OVERdrive, where it easily gets 42MPG. That's just the way it was designed. If I downshift it to drive, the most fuel-efficient 3500RPM/43%(redline)/.6(of max horsepower RPM) equates to exactly 55MPH. I have no idea how fuel-efficient it is at 55MPH in drive, as there is nowhere near me where I could safely test that. Even the highways posted at 55MPH have average traffic speeds well in excess of 70MPH, and it's just not a good idea to hold a steady speed 20MPH below the flow in any lane. BUT, I suspect my vehicle would be quite fuel-efficient at 55MPH, as that is how it was designed. The most fuel-efficient RPM of my engine happens to fall right AT 55MPH, when the car is in drive gear. No way was THAT a coincidence, so it had to be by design. Yet since I learned how to find the most fuel-efficient RPM of an engine, I have discovered that a lot of vehicles are geared incorrectly. My wife's car is one of them. Her car should be most fuel-efficient at ~ 3400RPM, yet it is hard to get her engine to even hit 3000 (without driving fast enough to lose your license), which is the point of maximum torque. A lot of cars I've driven have been similarly geared incorrectly. In one extreme example, I drove a chevy with a 4-cylinder engine that rarely hit 2000RPM. It was horrible, as far as fuel economy goes. Now I know why. If "lugging" the engine was such a good idea, I'd expect to see more 7 or 8-speed trannies, both automatic and manual. That is, there should be more "over"drive gears to keep the engine running really slow. But if the car is geared correctly, only one overdrive gear is needed. This allows the car engine to max out the EPA estimate at 55MPH (where it is tested, in drive gear), and yet still get decent fuel economy closer to speeds that the car is actually driven. Thus, more overdrive gears would be redundant on a street-legal car. -Dave [snip: more efficiency vs. economy confusion...] >If "lugging" the engine was such a good idea, I'd expect to see more 7 or >8-speed trannies, both automatic and manual. In practice, only one cruising-only gear is required for good highway economy (which, as Steve correctly informed you, is achieved by lugging the engine at highway speeds). That's precisely how cars like late-model Corvettes can get close to 30 mpg with 300-400+ hp. The manual trans cars are geared for ~40 mph/1000 rpm in top gear, which allows just enough surplus power to climb most freeway grades at normal fuel mixtures without downshifting. That top gear is useless for any purpose other than maximizing highway fuel economy, but it serves that purpose very well. Say, have you been able to figure out what kind of mpg a Corvette should get at 125 mph in 6th? According to your bogus (40% of redline/60% of power peak/x% of whatever) rpm-based rule, that should be the sweet spot for fuel economy, right? Whaddya think? 40 mpg at 125 mph? Maybe one day, you'll allow yourself to learn the difference between engine efficiency and fuel economy, but until then, I guess you'll have to consider Corvette fuel economy to be one of those weird Mysteries of the Universe. -- Chuck Tomlinson >>The implicit assumption in that sort of "rule of thumb" is that you NEED >>all the power that the engine can produce. Its a good rule for, say, an [quoted text clipped - 4 lines] > > Ummmmm, no. Think what you want, don't let facts stand in your way. The most fuel-efficient RPM of an Otto Cycle engine is well > below the RPM that will produce the most horsepower. Uh, that's what I SAID. "Lugging" the engine > is not fuel-efficient, as the engine RPM is too low. That contradicts what YOU just said! > At low RPM, too much > energy is wasted as heat. (a lesser percentage of energy is converted to > kinetic energy while the engine is lugging) And that's simple bullcrap. > Overdrive is meant to improve > fuel efficiency at higher speeds. (OVER drive speeds) It can't do this by > "lugging" the engine. It does this by reducing the RPM into a more > fuel-efficient range while still NOT lugging it. Wordsmith the definition of "lug" however you want to, but it doesn't change anything. > Good example: My car is > at 3500RPM/43%(redline) and .6 (of max horsepower RPM) at 78MPH in > OVERdrive, where it easily gets 42MPG. Good grief, man! that's not even CLOSE to being a typical modern overdrive gear. A typical overdrive gear is a lot more like what I originally stated- say 1800 RPM at 70 MPH on a 5500 RPM redline engine. > Good grief, man! that's not even CLOSE to being a typical modern > overdrive gear. A typical overdrive gear is a lot more like what I > originally stated- say 1800 RPM at 70 MPH on a 5500 RPM redline engine. That must be a big V8 engine. My 3.5 liter V6 does 2350 RPM (and gets around 33 mpg) when cruising at 75 mph on a level road. And redline is at 6600. >> Good grief, man! that's not even CLOSE to being a typical modern >> overdrive gear. A typical overdrive gear is a lot more like what I [quoted text clipped - 3 lines] > around > 33 mpg) when cruising at 75 mph on a level road. And redline is at 6600. Ford Expedition, 5.4L Triton. I'd call that a "medium" v8... but then to me a 440 Mopar, 460 Ford, or Caddy 500 is what qualifies as "big v8" :-) Checked the wife's 3.5 (Chrysler) engine today- I drove it to have the tires rotated. 2100 RPM at 70. Redline 6500 >>> Good grief, man! that's not even CLOSE to being a typical modern >>> overdrive gear. A typical overdrive gear is a lot more like what I >>> originally stated- say 1800 RPM at 70 MPH on a 5500 RPM redline engine. >> That must be a big V8 engine. My 3.5 liter V6 does 2350 RPM (and gets >> around >> 33 mpg) when cruising at 75 mph on a level road. And redline is at 6600. > Checked the wife's 3.5 (Chrysler) engine today- I drove it to have the > tires rotated. 2100 RPM at 70. Redline 6500 I wonder why Audis have such high top gear ratios. At 70 mph in 6th, the engine is running at 2800 rpm. > I wonder why Audis have such high top gear ratios. At 70 mph in 6th, > the engine is running at 2800 rpm. Smaller engine, I suspect. What's the capacity? And how many MPG are you getting? > Smaller engine, I suspect. What's the capacity? And how many MPG are you > getting? 2.7L V6 twin turbo. I get roughly 25 to 26 mpg on average according to the trip computer. On Sun, 18 Sep 2005 07:52:40 GMT, TLittle <mail@mail.net> said the following in rec.autos.driving... <snip> > What's the point of the tachometer on a vehicle with an automatic > transmission? > > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? Often wondered about this myself. If the auto makers just have some bug up their butts that says they *have* to put gauges in the instrument cluster, I can think of several that I would rather have before a tach. SignaturePaul. Self appointed unofficial overseer of kooks and trolls in rec.autos.driving > Curious as hell about this. > [quoted text clipped - 19 lines] > Seriously, based on my understanding of WHY the tachometer exists, why > are there tachs on vehicles with automatic transmissions? They are little more than visual entertainment for virtually all drivers. You can watch the needle move up and down as the automatic transmission shifts itself. For those who are truly bored or need to feel in control a tiptronic type automatic transmission in combination with a tach can provide hours of road-racer-like thrills. The space could be used more effectively with a good analog clock and a couple of useful gauges like oil pressure and voltage. > They are little more than visual entertainment for virtually all > drivers. You can watch the needle move up and down as the automatic [quoted text clipped - 4 lines] > The space could be used more effectively with a good analog clock and > a couple of useful gauges like oil pressure and voltage. While I don't disagree entirely; a tach is not completely useless. When I'm sitting a a light with the stereo up and the car is idling a little rough the tach might indicate that I'm only idling at 400 rpm. If I'm at the same light and the car is idling exceptionally smooth the tach might indicate that the engine has died completely. Until I recently diagnosed a faulty idle speed stepper motor either of these was a regular occurrance. TLittle <mail@mail.net> writes in article <432D1CDA.E029CA73@mail.net> dated Sun, 18 Sep 2005 07:52:40 GMT: >What's the point of the tachometer on a vehicle with an automatic >transmission? So you can detect a stall at a traffic light without turning the stereo down. :^) >Seriously, based on my understanding of WHY the tachometer exists, why >are there tachs on vehicles with automatic transmissions? Most automatic transmissions have low-gear settings. It would be a good thing to know when you're at the upper range of one of these. -- spud_demon -at- thundermaker.net The above may not (yet) represent the opinions of my employer. > TLittle <mail@mail.net> writes in article <432D1CDA.E029CA73@mail.net> dated Sun, 18 Sep 2005 07:52:40 GMT: > >What's the point of the tachometer on a vehicle with an automatic [quoted text clipped - 7 lines] > > Most automatic transmissions have low-gear settings. Yes, most automatic and manual transmissions have a low gear setting. > It would be a good > thing to know when you're at the upper range of one of these. An automatic transmission takes care of that automatically. > -- spud_demon -at- thundermaker.net > The above may not (yet) represent the opinions of my employer. > An automatic transmission takes care of that automatically. You do know that all auto trannies allow you to shift down, right? -Dave > > An automatic transmission takes care of that automatically. > > You do know that all auto trannies allow you to shift down, right? -Dave Yes, as do manual transmissions. And your point is...... >> You do know that all auto trannies allow you to shift down, right? -Dave > > Yes, as do manual transmissions. And your point is...... That RPMs can get awfully darn high when you downshift. While a tachometer isn't absolutely necessary (regardless of transmission), it is useful (regardless of transmission) -Dave > >> You do know that all auto trannies allow you to shift down, right? -Dave > > [quoted text clipped - 3 lines] > isn't absolutely necessary (regardless of transmission), it is useful > (regardless of transmission) -Dave Please don't take this personally, but if you don't have a good idea what will happen to engine speed when you downshift a tachometer will not do much to solve that problem. If you drop it into an inappropriately low gear the tach will only tell you after the fact that you screwed up and overrevved the engine. Engine noise will tell you roughly the same thing. I would strongly suggest that you just leave the transmission in Drive and let brakes do the work of slowing the car. haven't read the entire thread but looks like this is the first "correct" answer: When descending a hill, with the automatic transmission selector lever in low gear, like "L" or "2", the tachometer can be used to insure you do not exceed redline and incur engine damage, which when locked manually in the lower gear is possible. > haven't read the entire thread but looks like this is the first > "correct" answer: [quoted text clipped - 3 lines] > do not exceed redline and incur engine damage, which when locked > manually in the lower gear is possible. True for older purely hydro-mechanical transmissions. But name me an automatic transmission currently made that won't upshift at or before redline REGARDLESS of what gear is selected. There are probably a few still out there, but not many. > haven't read the entire thread but looks like this is the first > "correct" answer: [quoted text clipped - 3 lines] > do not exceed redline and incur engine damage, which when locked > manually in the lower gear is possible. If you don't know what will happen prior to moving the shift lever a tachometer won't solve that problem. You would be much better off leaving the car in drive if you can't avoid overrevving an engine by dropping to the next lower gear. A tach is for the most part low grade visual entertainment - watch the moving needle. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type"> <title></title> </head> <body bgcolor="#ffffff" text="#000000"> <blockquote cite="middh9lt4$3ad$2@newslocal.mitre.org" type="cite"> <pre wrap="">TLittle <a class="moz-txt-link-rfc2396E" href="mailto:mail@mail.net"><mail@mail.net></a> writes in article <a class="moz-txt-link-rfc2396E" href="mailto:432D1CDA.E029CA73@mail.net"><432D1CDA.E029CA73@mail.net></a> dated Sun, 18 Sep 2005 07:52:40 GMT: </pre> <blockquote type="cite"> <pre wrap="">What's the point of the tachometer on a vehicle with an automatic transmission? </pre> </blockquote> <pre wrap=""><!----> So you can detect a stall at a traffic light without turning the stereo down. :^) </pre> <blockquote type="cite"> <pre wrap="">Seriously, based on my understanding of WHY the tachometer exists, why are there tachs on vehicles with automatic transmissions? </pre> </blockquote> <pre wrap=""><!----> Most automatic transmissions have low-gear settings. It would be a good thing to know when you're at the upper range of one of these. -- spud_demon -at- thundermaker.net The above may not (yet) represent the opinions of my employer. </pre> </blockquote> I offer the following; it hasn't to do with why or when to use a tach with an automatic transmission, but rather why there's a tach in an automobile with an automatic transmission at all.<br> <br> It's more cost effective for the manufacturer to include a tach in all models where a tach would otherwise be an option. As I postulate, it's cost effective from the design, manufacturing, procurement, assembly, and all other aspects for the manufacturer, not necessarily the consumer. All consumers pay a small price for a gauge few will ever use, even those with a manual transmission.<br> <br> This is especially apparent when you consider the similarities (i.e., cost effectiveness) among models, not only with the gauges, but also the supporting electronics as well.<br> <br> Similar to, but different than...why are automatic transmissions even offered in certain vehicles, for example: Jeep Wrangler, Corvette, and 4x4 Trucks. I already know the answer, but it still makes me laugh a bit when I pull up next to a "sports car" and look down and see an automatic shifter.<br> <br> <div align="left"><i><font color="#009900">Agave</font></i><br> </div> <i><font color="#009900">Salud, dinero, amor y tiempo para disfrutarlos<br> 01 F350 PSD Crew Dually</font></i><br> </body> </html> Here's another idea. After cleaning and adjusting throttle body, removing and plugging a vacuum line on the throttle stop, the final adjustment calls for holding engine speed at 2500 rpm, then releasing the throttle and checking the engine speed, then loosening a small locknut and adjusting. Tachometer was helpful. > It's more cost effective for the manufacturer to include a tach in all > models where a tach would otherwise be an option. But there are many models that don't offer ANY manual transmission option AT ALL, and yet they have tachs. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type"> <title></title> </head> <body bgcolor="#ffffff" text="#000000"> <blockquote cite="midw8udnQZ_xIMB1NzeRVn-pA@texas.net" type="cite">Agave wrote: <br> <br> <blockquote type="cite">It's more cost effective for the manufacturer to include a tach in all models where a tach would otherwise be an option. </blockquote> <br> But there are many models that don't offer ANY manual transmission option AT ALL, and yet they have tachs. <br> </blockquote> In keeping with my prior opinion, consider not just a specif model, but also...<br> <ul> <li>"similar" models, for example the Taurus and Sable</li> <li>as well as the Canadian, European, and other market varieties</li> <li>internationalization of auto manufacturers, for example Ford and Mazda - the Probe and MX-6</li> <li>cross model pollinization <br> </li> <li>a US model may not offer a manual transmission, but an international market model may</li> </ul> I'd argue that very few people actually need a tach, regardless of transmission type. As others have pointed out, with most new cars/trucks having rev limiters, a tach is a "nice thing to watch", but not very useful, as well as how many people and how often does one red line their engine.<br> <br> I think you can blame market research, focus groups, product testing, grabbing a piece of the after market pie, business decisions (e.g., "I'll give you a deal on 10 million fuel gauges, if you buy 3 million tachometers."), and just the general American mind-set of wanting things you don't really need for the reasons why there are tachs in cars with automatic transmissions.<br> <br> At this point though, I think most people would notice that they didn't have a tach...even though they might not have a clue what it does :). I commend you for noticing just the opposite and questioning the meaning of things.<br> <br> Now, if someone can tell me why people don't use their cruise control, I'd be interested.<br> </body> </html> > It's more cost effective for the manufacturer to include a tach in all models where a tach would otherwise be an option. > But there are many models that don't offer ANY manual transmission option AT ALL, and yet they have tachs. [quoted text clipped - 12 lines] > > Now, if someone can tell me why people don't use their cruise control, I'd be interested. Most drivers push on the gas and the electronic transmission shifts properly and does what it is supposed to and little attention is given to shift points. The tach becomes for most people a bit of moving eye candy as the needle goes up and down betweeen 2,200 and 2,900 rpm in response to automatic gear changes. The very limited space available in most cars could be more effectively used to display a couple of truly useful gauges and a small analog clock. Most of us really don't need (or know how) to monitor rpm's and time shifts accordingly. Ditching it may not be so easy because the tach has become such an automotive fixture that if it were eliminated it would be seen as step backwards. >Most drivers push on the gas and the electronic transmission shifts >properly and does what it is supposed to and little attention is given >to shift points. The tach becomes for most people a bit of moving eye >candy as the needle goes up and down betweeen 2,200 and 2,900 rpm in >response to automatic gear changes. I have never known this to be the case. In fact, I was driving a rented Chrysler Sebring last week. Found an opening into traffic and punched the pedal hard. The tachometer whizzed up, as the engine also whizzed with a sound like a sewing machine out of control... but the car did not accelerate whatsoever for a good second after the pedal was pressed. Is this normal? It's been more than a decade since I last drove an automatic. Do people really put up with this? >The very limited space available in most cars could be more effectively >used to display a couple of truly useful gauges and a small analog >clock. Most of us really don't need (or know how) to monitor rpm's and >time shifts accordingly. Ditching it may not be so easy because the >tach has become such an automotive fixture that if it were eliminated >it would be seen as step backwards. I would agree that oil pressure, temperature, and maybe battery voltage meters would be more useful than a tach... however we are entering a world where these things are either being removed or replaced with fake meters that give only a go/no-go indication because it is feared they will confuse the driver. Even BMW is now selling cars with temperature gauges that read cold, hot, and nominal, with no real steps in-between. This is horrifying. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." > >Most drivers push on the gas and the electronic transmission shifts > >properly and does what it is supposed to and little attention is given [quoted text clipped - 10 lines] > Is this normal? It's been more than a decade since I last drove an > automatic. Do people really put up with this? Can't tell you about that specific car, but it sounds like a mechanical problem and not one that a tachometer could solve. If it really took a full second for the car to begin any acceleration at all then there is a problem with that vehicle. The numerous car's I've driven over the years have all been responsive when consideration is given to the engine and size of car involved. A Hertz chevy with a low-end v-6 won't have the same punch as a turbo-charged Volvo S80 T6. Both will accelerate when the gas is pushed however. > >The very limited space available in most cars could be more effectively > >used to display a couple of truly useful gauges and a small analog [quoted text clipped - 9 lines] > the driver. Even BMW is now selling cars with temperature gauges that read > cold, hot, and nominal, with no real steps in-between. This is horrifying. I test drove a Mercedes Benz about 4 years ago that had the poorest design for instrumentation I had ever seen. This car used a series of digital displays to show trip meter, temperature, etc. Truly a distracting and potentially dangerous design because you had to work through a couple of menus to find information that should be available at a glance while driving down the road. > --scott >>Most drivers push on the gas and the electronic transmission shifts >>properly and does what it is supposed to and little attention is given [quoted text clipped - 10 lines] > Is this normal? It's been more than a decade since I last drove an > automatic. Do people really put up with this? In my experience most cars with auto trannies do this, although of course some are better than others. One reason why I hate AT is because of this. My husband has a Saturn which is an ok car, but it does this. When he's driving and punches it to get around traffic, I usually can't resist a snicker along with a comment like "the hamster's running as fast as he can, poor thing", implying he's got a hamster in a cage instead of a combustion engine. I'm no perfect driver by any means, but I'm still better at timing my shifts (in my manual Dakota) than his auto tranny is. I've also noticed a habit many AT drivers have, of punching it and when the engine finally shifts, they let off the accelerator and slow down, thus rendering the whole exercise meaningless. - Sharon "Gravity... is a harsh mistress!" > I've also noticed a habit many AT drivers have, of punching it and when > the engine finally shifts, they let off the accelerator and slow down, > thus > rendering the whole exercise meaningless. > > - Sharon No, that is an improperly designed transmission. I drive a manual tranny very well. The main thing that annoys ME about automatics is that every single one of them I've driven (dozens) are designed to shift up WAY too soon. You can adjust the upshift point somewhat by nailing the accelerator to the floor. Even if you do that though, the tranny will STILL upshift too soon. Or, if you are going slow enough, you could manually shift the auto tranny down to "2" or "1" until you are ready to let it upshift again. But if you have to put so much thought and effort into making the tranny shift right, what's the point of owning an automatic, exactly? The manual tranny is easier to control, if you HAVE to control it. My point? Many of these AT drivers you observe, who you thought were letting off the accelerator, probably were not letting off the accelerator. More likely, they applied steady pressure to it. It is the *^*(*&#( automatic tranny that downshifts for about a half-second only to upshift again, unless the driver takes extraordinary measures to prevent such foolishness. Yeah, there are some automatic trannies designed to be shifted often. Sometimes there's buttons on the shifter or even steering wheel, and sometimes the shifter itself can be moved into different gears, kind of like a clutchless manual. Call them manumatics or something. :) What's the point? Just buy a car with a clutch, so it will do what it's supposed to do, without having to jump through hoops to make it shift correctly. -Dave > > I've also noticed a habit many AT drivers have, of punching it and when > > the engine finally shifts, they let off the accelerator and slow down, [quoted text clipped - 7 lines] > single one of them I've driven (dozens) are designed to shift up WAY too > soon. Way too soon for...???? > You can adjust the upshift point somewhat by nailing the accelerator to the > floor. Even if you do that though, the tranny will STILL upshift too soon. [quoted text clipped - 3 lines] > what's the point of owning an automatic, exactly? The manual tranny is > easier to control, if you HAVE to control it. Most automatic transmissions are adaptive and will modify shift points to an extent. If someone wants to play race driver with an automatic transmission then get one with a Tiptronic-like adaptor. Several brands have them. In my experience most drivers of manual transmssion cars really don't know how to maximize milage or performance however and would be better off with an automatic. Most manual transmission drivers I've seen either hold their car in too low a gear and waste gas like crazy or don't know when to downshift. It's a disappearing art unfortunately. One more point, many automatic transmission are easy to control. I can force mine to start in first, second or third gear as needed for traction, and can hold shifts for downhill driving, etc. > My point? Many of these AT drivers you observe, who you thought were > letting off the accelerator, probably were not letting off the accelerator. [quoted text clipped - 9 lines] > point? Just buy a car with a clutch, so it will do what it's supposed to > do, without having to jump through hoops to make it shift correctly. -Dave > > > I've also noticed a habit many AT drivers have, of punching it and when > > > the engine finally shifts, they let off the accelerator and slow down, [quoted text clipped - 9 lines] > > Way too soon for...???? Well I would think that would be obvious. Most automatic trannies upshift long before the engine enters the powerband. Or worse, the engine will just barely TOUCH the powerband, and then the tranny upshifts immediately. You want acceleration with an automatic? Better get out and push. It's even more annoying driving an automatic on hills. It will downshift properly to climb a hill, but then upshift again right away, while the car is still trying to climb the hill. It is bizarre the way an automatic shifts, if you know how to drive a manual. Yes, there are workarounds, but driving a manual tranny is actually easier than trying to manually control a lushbox. -Dave > > > > I've also noticed a habit many AT drivers have, of punching it and > when [quoted text clipped - 14 lines] > Well I would think that would be obvious. Most automatic trannies upshift > long before the engine enters the powerband. Now that is an authoritative sounding but truly misleading statement. Since the power or torque curve is just that...a curve that begins at zero, any time the transmission shifts above zero it will be someplace in the power band. Its a question of where on the curve you want to shift. In addition to controlling quickly you can accelerate, the place you select on the curve will also govern how much fuel you burn and how much wear and stress you place on the engine. You appear to have little concern about fuel economy and engine longevity - playing the race driver seems to be your focus. > Or worse, the engine will just > barely TOUCH the powerband, and then the tranny upshifts immediately. You > want acceleration with an automatic? Better get out and push. It's even > more annoying driving an automatic on hills. It will downshift properly to > climb a hill, but then upshift again right away, while the car is still > trying to climb the hill. I have yet to experience that in hundreds of thousands of miles driving four cars equipped with automatic transmissions. > It is bizarre the way an automatic shifts, if you > know how to drive a manual. Interesting but rather bizarre statement that is not supported by the real world of driving that I've encountered. I drove numerous stick shifts before switching to an automatic several cars ago. The electronically controlled automatic of today does a far better job of shifting than most drivers I've encountered. Most guys with manual shift transmissions seem to like keeping the engine in a lower gear than needed presumably for the imagined race car thrills from hearing the engine at 4,000 rpm. Whee...look everyone...I'm a race car driver 'cause I got me a car with a five speed, tach, spoiler and noisy exhausts to prove it. Yes, there are workarounds, but driving a > manual tranny is actually easier than trying to manually control a > lushbox. A lushbox...??? A box full of alchoholics maybe???? The following service procedures made mine a great deal more responsive, overcoming the characteristics you describe to the point that the transmission will hold the gears for better acceleration when desired to the point of maximum safe engine speed, downshift quickly when appropriate, and upshift early under light acceleration to maximize fuel economy. 1) put a piece of 2x4 and a concrete block to hold the gas pedal to the floor, then check the throttle butterfly valve for full opening at WOT. I found a cable adjustment that needed to move apx. 1/4". The transmission shift cable was also linked to the same assembly, so once the cable was adjusted, the trans.cable was fine. 2) factory service manual lists a number of procedures for cleaning the throttle body and adjusting the throttle position sensor (TPS) and I performed all of them with the aid of a digital multimeter including an audile continuity test. The TPS position was off by a few thousandths. Once the IDL stop was set correctly, the reading at WOT was fine. 3) there are some tiny orofices in the throttle body that direct vacuum to the EGR vacuum modulator. After following directions and cleaning with carburetor cleaner, then blowing out the passages with compressed air, one of the smaller passages emitted a burst of dark colored fluid - presumably gum had been occluding that port to some degree if not completely blocking it. 4) adjust throttle stop at vacuum dashpot with vacuum line removed according to directions. As stated, transmission now shifts exactly as one would wish, although it has taken practice to learn the throttle mapping, so as to be able to feather the downshift points to activate or not as desired - roughly between 50 - 60 per cent throttle opening. (snip) >brands have them. In my experience most drivers of manual transmssion >cars really don't know how to maximize milage or performance however >and would be better off with an automatic. Most manual transmission >drivers I've seen either hold their car in too low a gear and waste gas >like crazy or don't know when to downshift. It's a disappearing art >unfortunately. (snip) One thing I wish I had is a torque-speed curve and fuel efficiency map and whatever for my engine. I don't know where its most efficient points are, partly because I can't find that information out. Are there any good sources of this type of advice? My gears are set fairly close together so I have plenty of choice about RPM, I'm just not usually sure what the best option is. Normally for just cruising along at a steady speed I choose a gear that has the engine at between 2,000 and 2,500 RPM, but I'm far from confident that that's a good choice. I guess it varies from model to model. -- Mark > One thing I wish I had is a torque-speed curve and fuel efficiency map > and whatever for my engine. I don't know where its most efficient [quoted text clipped - 5 lines] > and 2,500 RPM, but I'm far from confident that that's a good choice. I > guess it varies from model to model. A couple of rules of thumb, gathered from various sources that know a lot more about car engines than I ever will: Aim for 40-45% of engine redline Aim for RPM at (60% of RPM at which engine produces maximum horsepower) In several cars I've done the math on, these two formulas produce results pretty close to each other. For my own car, it works out to (3200 to 3600RPM) or 3480RPM, depending on which formula I use. Over ten years of driving this car, I have noticed that maximum fuel economy is achieved at 3500 RPM. Not coincidentally, my engine is most efficient right where it should be, according to the experts. So use either of the above formulas, and you should be pretty close to where you want to be. If you can't find the horsepower/torque curves, you should at least be able to dig up the manufacturer's published horsepower specs. for the engine, showing (for example) 150HP @ 5800RPM or whatever your engine produces at X rpm. They always list this by maximum horsepower RPM, so just multiply that RPM by .6 to get a pretty good estimate of what to aim for. Unfortunately, it might not be that easy if you own a slushbox. :( But I gathered from what you wrote that you have a real tranny, so you should be all set. -Dave > A couple of rules of thumb, gathered from various sources that know a > lot more about car engines than I ever will: [quoted text clipped - 3 lines] > Aim for RPM at (60% of RPM at which engine produces maximum > horsepower) Are you talking about shift-points or cruising? > In several cars I've done the math on, these two formulas produce > results pretty close to each other. For my own car, it works out to > (3200 to 3600RPM) or 3480RPM, depending on which formula I use. Over > ten years of driving this car, I have noticed that maximum fuel > economy is achieved at 3500 RPM. [snip] WTF! In my car 3500 RPM puts me at over 80 MPH. What exactly are you driving? Based on what you're saying here I would seldom get my car out of second gear. > Are you talking about shift-points or cruising? > [quoted text clipped - 6 lines] > WTF! In my car 3500 RPM puts me at over 80 MPH. What exactly are you driving? > Based on what you're saying here I would seldom get my car out of second gear. Cruising. Yes, some cars are designed for efficient high-speed cruising. I thought my own car was one of them, until I realized that I was in OVER-drive. Shifting down to drive put me at 3500RPM at (ta daaaaa!!!!) 55MPH. So my car was designed to be most efficient at 55MPH. That makes perfect sense, as it has to meet CAFE requirements. I don't know about your car, but in over-drive, my most fuel-efficient engine RPM is at 78MPH. While I doubt your car would be most fuel-efficient at somewhre over 80MPH, it wouldn't terribly shock me, if that was true. I've owned several cars myself that were most fuel-efficient somewhere above 70MPH. Many people have observed that their own cars are most fuel-efficient at similar speeds. This makes perfect sense. If you designed a car to be most fuel-efficient at 55MPH in over-drive, it would have a 0-60 time measured in minutes, and nobody would buy it. So it's not surprising at all that many cars are most fuel-efficient at speeds above 55MPH in over-drive. -Dave I noticed on a vehicle years ago with a automatic overdrive transmission which will dramatically lower engine rpm to about 2000 to 2200 rpm at 60 to 65 mph at a true longer distance cruising speed outside the outer belt in rural areas. I read these on a C body Coupe DeVille Cadillac with both (BCM) body control module and (ECM) engine control module engineered with built in on-board diagnostics with a FWD THM440 xmission and HT4100 V8 transverse engine. The year is not important. Before the days of Interstates, the best fuel economy was at about 35 mph due to urban and rural driving habits at $ .12 to $ .17 per gallon. Very few vehicles had overdrive transmissions, a Uncle once stated a 82 hp. 1939 Plymouth Coupe during WWII he owned got 20 to 21 mpg driving from Columbus OH to Chanute Field in "Illinois" at 50 to 55 mph. This Plymouth Coupe had a flathead 6 engine with a 1 barrel carb, leaded fuel, 3 speed manual with a 4.11 rear end ratio, and 6.00 by 16 bias ply tires!!!!! Imagine the true meaning of the "KISS" (keep it simple stupid" principle with NO fuel injection controlled by onboard computer, NO catalytic converter, NO AOD transmission problems, and a $ 845 car FOB Destination delivered. snip > I noticed on a vehicle years ago with a automatic overdrive transmission > which will dramatically lower engine rpm to about 2000 to 2200 rpm at 60 [quoted text clipped - 22 lines] > > snip Most cars today use torque converter lockups to bring rpms down to around 2,000 at 60mph and be reasoably fuel efficient. Most manual transmissions today have a built overdriven 5th gear if that is of any help. Whether the car uses an overdrive or not is not as important to fuel economy as how factrs like the overall drive ratio, number of gears available, the circumference of the wheels, the engine power and size of the vehicle are tied together. >> In several cars I've done the math on, these two formulas produce >> results pretty close to each other. For my own car, it works out to [quoted text clipped - 5 lines] > >Based on what you're saying here I would seldom get my car out of second gear. This topic was recently discussed at length in r.a.d. In a nutshell, Dave C doesn't understand the difference between engine fuel efficiency (e.g., power out / fuel flow rate) and vehicle fuel economy (e.g., miles / gallon used). He is under the delusion that the engine's most efficient operating rpm will coincide with the vehicle's best fuel economy speed. I'm still waiting for him to estimate my car's fuel economy at 125 mph (at his recommended rpm). If you follow his extremely poor advice, the result will be excessive cruising fuel consumption. If you drive a modern car (roughly 1990-up, but true for most older cars too) in good running condition, save fuel by cruising in top gear when possible. Simple. -- Chuck Tomlinson (snip) >This topic was recently discussed at length in r.a.d. In a Ah, thanks - I'll check the archives on Google. >nutshell, Dave C doesn't understand the difference between >engine fuel efficiency (e.g., power out / fuel flow rate) >and vehicle fuel economy (e.g., miles / gallon used). Mmmm - I remember my high school math teacher once made the same mistake in a problem the class worked on! (snip) >If you follow his extremely poor advice, the result will be >excessive cruising fuel consumption. Also, John mentions engine wear, another worrying issue. >If you drive a modern car (roughly 1990-up, but true for >most older cars too) in good running condition, save fuel by >cruising in top gear when possible. Simple. That is simple. (-: It also matches what I'm used to others doing. Thanks. -- Mark > > One thing I wish I had is a torque-speed curve and fuel efficiency map > > and whatever for my engine. I don't know where its most efficient [quoted text clipped - 10 lines] > > Aim for 40-45% of engine redline > Aim for RPM at (60% of RPM at which engine produces maximum horsepower) OK, I have to ask what the goal is by maintaining an artificially high rpm all the time. If you are running along at 3200 or 3600 rpm when you could be in a higher gear you are wasting gas and inducing more wear on the engine. > In several cars I've done the math on, these two formulas produce results > pretty close to each other. For my own car, it works out to (3200 to [quoted text clipped - 3 lines] > should be, according to the experts. So use either of the above formulas, > and you should be pretty close to where you want to be. You say that the formulas will get you pretty close to where you want to be, but what will that level of rpm result in. If you are cruising down the highway at 60mph in 3rd gear to maintain 3,600 rpm what is being accomplished. There was an urban legend that lasted for decades about Porsche engines having to be run at 3600 rpm or higher all the time or the lower end would fall out. Lots of guys wore those boxer engines out prematurely as a result. If you can't find > the horsepower/torque curves, you should at least be able to dig up the > manufacturer's published horsepower specs. for the engine, showing (for [quoted text clipped - 5 lines] > gathered from what you wrote that you have a real tranny, so you should be > all set. -Dave > > Aim for RPM at (60% of RPM at which engine produces maximum horsepower) > > OK, I have to ask what the goal is by maintaining an artificially high > rpm all the time. If you are running along at 3200 or 3600 rpm when > you could be in a higher gear you are wasting gas and inducing more > wear on the engine. Tell that to your local legislators who thought it necessary to mandate that you waste fuel in order to minimize safety and maximize revenue to the local governments and maximize carnage on our highways. In other words, you are looking at this wrong. I'd gladly upshift and drive safely at the most fuel-efficient speed of my engine in overdrive gear, if it was legal for me to do so. [Often, I do so anyway. :)] But at 78MPH, I am risking my license by driving in a very safe, sane, logical and fuel-efficient manner in over-drive. Yes, I could do 55MPH in 4th gear everywhere. I would get decent fuel economy that way. But I'd have to put up with the extra engine noise generated at 3500RPM, while it's taking forever and a day to get anywhere. Plus, it's harder to hold a constant speed in 4th gear. Because I'm lazy, I generally shift up to overdrive even at relatively low speeds like 55MPH. But then, I pay for my laziness at the gas pump. The only time I get really good gas mileage is when I'm in a hurry and not as worried about getting hassled by the cops. IF I can maintain an average speed close to 78MPH for long enough, my MPGs go way up. But again, that's not exactly legal in most areas of the U.S. -Dave > > > Aim for RPM at (60% of RPM at which engine produces maximum horsepower) > > [quoted text clipped - 11 lines] > license by driving in a very safe, sane, logical and fuel-efficient manner > in over-drive. What the hell does 78mph have |
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