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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / May 2008MPG gauge.
Some new cars have digital mpg sensors and gauges. For those of you that have them, how accurate do you think they are? When on interstate how much difference is their in mpg at 55 mph, 65 mph, and 65 mph? A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg on stop and go gravel back roads. > Some new cars have digital mpg sensors and gauges. For those of you > that have them, how accurate do you think they are? [quoted text clipped - 4 lines] > A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg > on stop and go gravel back roads. Most of todays cars have computers that do the calculations, using fuel flow to calculate the percentages in finite detail. They're pretty accurate... For instance, I own a 300Hp V6, sedan ( around 4000 #). On Interstate highways, at 55, I get 27.5 mpg, 60 gives around 26, 65 around 25, etc.. A couple years ago, I rented a big caddy at Salt Lake City to drive to Reno, Tahoe, etc. This was the first year they mentioned $4.00/gal gas.. Being a bit cautious, I started out westbound on US 80 toward Bonneville and would only drive around 60 mph (or less), but I was sightseeing too. I thought the fuel calculations were way off, but more than once, the readout said 32mpg and it proved true when I filled up. On the return, I kept the pedal down, to make my plane out of SLC and it still got over 28. Of course this is on mostly, very flat terrain. Much of your MPG will depend on your driving habits.. > Some new cars have digital mpg sensors and gauges. Actually most new cars either have them as stanrd or as an option now. > For those of you > that have them, how accurate do you think they are? > > When on interstate how much difference is their in mpg at 55 mph, 65 > mph, and 65 mph? Within those ranges MPG will go down down as MPH goes up. That is pretty much a given. > A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg > on stop and go gravel back roads. That makes sense to me. MPG computers are very acccurate in my experience if you measure average mpg over a full tank. The spot number also appears to be quite acccurate. > > Some new cars have digital mpg sensors and gauges. > [quoted text clipped - 17 lines] > average mpg over a full tank. The spot number also appears to be > quite acccurate. The spot number also gives you a pretty good idea of how much gas certain bad driving habits cost you.  SignaturePaul Hovnanian paul@hovnanian.com ----------------------------------------------------------------------- Procrastinators: The leaders for tomorrow. > > > Some new cars have digital mpg sensors and gauges. > [quoted text clipped - 27 lines] > > - Show quoted text - It sure does. And it's a great tool for learning tricks to maximize mpg too. > > MPG computers are very acccurate in my experience if you measure > > average mpg over a full tank. The spot number also appears to be > > quite acccurate. > > The spot number also gives you a pretty good idea of how much gas > certain bad driving habits cost you. It sure does. And it's a great tool for learning tricks to maximize mpg too. ==== When I borrowed a friend's Golf TDI, which has an MPG meter, it was interesting to see what did and didn't affect the reading. - at a constant speed, if I relaxed the accelerator even very slightly, the MPG figure shot up to amazingly good figures (eg 80-90); if I pressed the accelerator even slightly beyond the amount needed to keep a constant speed, it went down to about 40; at a steady 60 mph I was getting about 55 mpg (all thse figures are per *UK* gallon: multiply by 4/5 to get US mpg) - the consumption at a given speed seemed to be about the same irrespective of whether the engine was hot (running for a long time) or cold (newly started). - changing up or down made surprisingly little difference: 50 in 4th or 4th gear gave similar results with 5th only being slightly more efficient than 4th; however changing down in to 3rd (with the engine going at about 4000 rpm) was much less efficient, so there seems to be a cut-off engine speed below which consumption is fairly constant but above which consumption increases sharply (probably both of the last two points are features of a diesel which might not be true for a petrol engine) - hard acceleration gave frighteningly low figures: a fairly brisk but not wheel-spinning 0-60 through the gears, taking about 15 seconds, averaged about 15 mpg :-( - constant 30 was much much more efficient than slowing down to rest for a junction and then accelerating away again; however as I came right off the power when I was slowing down, I did get something like 300 mpg ;-) - accelerating out of a roundabout in 3rd with a wider throttle setting was more efficient than in 2nd with the engine going faster, for similar acceleration rates - again, probably a diesel thing > It sure does. And it's a great tool for learning tricks to maximize > mpg too. A manifold vacuum gauge is also a pretty handy tool for "training" you to use less fuel. Lower manifold pressure generally equates to higher mileage, unless carried to the point of idling away from traffic lights... ;-) >>> Some new cars have digital mpg sensors and gauges. >> Actually most new cars either have them as stanrd or as an option now. >> >>> For those of you >>> that have them, how accurate do you think they are? In our Chevy Astro, we took an 8000mi trip. I used the records of fuel purchased during the trip and compared that with the total accumulated fuel that the computer claimed was burned. The comparison was (to me) surprisingly close. I don't recall the exact number, my recollection was that the numbers agreed within a percent or two. As far as I know, the computer makes all the fuel calculations (mpg, total fuel, etc.) based on injector pulse width. Even though the computer is mainly regulating fuel flow based on feedback from the oxygen sensor (and therefore extreme precision is not required), it seems to have a pretty accurate direct measure of fuel from pulse width and known fuel pressure. Of course, the accuracy of MPG is also affected by measurement of speed/distance; the primary uncertainty for that measurement is the tire diameter. Any changes from standard tires or axle ratio would require some sort of reprogramming to maintain accuracy. >>> When on interstate how much difference is their in mpg at 55 mph, 65 >>> mph, and 65 mph? [quoted text clipped - 11 lines] > The spot number also gives you a pretty good idea of how much gas > certain bad driving habits cost you. SignatureNOTE: to reply, remove all punctuation from email name field Ned Forrester n_f_orrester@whoi.edu 508-289-2226 Applied Ocean Physics and Engineering Dept. Oceanographic Systems Lab http://adcp.whoi.edu/ Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA >Some new cars have digital mpg sensors and gauges. For those of you >that have them, how accurate do you think they are? Those have been around in higher end cars since the advent of fuel injection. I have them on two current cars and find them to be pretty much right on the money. Steve B. > Those have been around in higher end cars since the advent of fuel > injection. I have them on two current cars and find them to be pretty > much right on the money. > > Steve B. We have one on our newish Avalon, and it seems to be rather accurate. They are very helpful, I think, if you are trying to optimize your driving habits with respect to fuel economy. It is also nice to have the estimated miles to empty feature, although I dont push that one very hard. > Some new cars have digital mpg sensors Its not a sensor. Its the engine management computer that does the math to come up with mileage. and gauges. For those of you > that have them, how accurate do you think they are? > [quoted text clipped - 3 lines] > A dump truck owner said his gauge reads about 5 mpg on highway, 1 mpg > on stop and go gravel back roads. The one on my wife's 1993 Chrysler 3.5L is EXTREMELY accurate. And it should be, after all the computer knows exactly how much fuel its putting through the injectors on every pulse, how far the car has gone, and how many pulses of what duration were required to get there. >> Some new cars have digital mpg sensors > [quoted text clipped - 17 lines] > gone, and how many pulses of what duration were required to get > there. Well actually, the PCM knows how long the injectors were commanded to be open. If you know this, the fuel pressure, the injector flow rate, and the miles traveled, you can estimate the mileage. However, over time both the fuel pressure and the injector flow rate can change and changes in tires can definitely affect the miles travel component of the estimation. I assume the manufacturers build in some compensation for changes, but I expect over time the estimate will vary. Cars I've owned with mileage meters generally overestimate the mileage slightly, but they still are pretty good. My current Ford Fusion overestimates the mileage by around 0.5 mpg if you compare the mileage calculated from gas purchases to the gauge estimate. Of course I am never sure exactly what the gauge estimate reflects - is it the average mpg since it was reset, or the mpg for the last X number of miles, or ? And my "calculated mileage is actually a little lower than actual since my odometer reads about 3% slow (I've actually gone 100 miles when it says I've gone 97). Ed >>> Some new cars have digital mpg sensors >> [quoted text clipped - 18 lines] > open. If you know this, the fuel pressure, the injector flow rate, and > the miles traveled, you can estimate the mileage. Yes, and all those ARE precisely known unless there's a system fault... and then you'd get an error code. Even though fuel pressure may not be measured directly by all EFI systems, it is correlated to manifold vacuum in a fixed, precise way, and manifold vacuum is measured. However, over time > both the fuel pressure and the injector flow rate can change and changes > in tires can definitely affect the miles travel component of the > estimation. Not much. Changing tire SIZE can have a big effect. Inflation- not so much. I assume the manufacturers build in some compensation for > changes, but I expect over time the estimate will vary. Cars I've owned > with mileage meters generally overestimate the mileage slightly, but > they still are pretty good. My current Ford Fusion overestimates the > mileage by around 0.5 mpg if you compare the mileage calculated from gas > purchases to the gauge estimate. I'd trust the built-in system more than I'd trust my ability to re-fill the tank to EXACTLY the same level on two given fill-ups. I'd want to average over 5-10 tank fulls before I'd claim that my measurement was more accurate than the built-in system.
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