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Car Forum / Volvo Cars / April 2006Gas mileage mystery
I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. Itseems that I get about 140 iles to the first half tank...the second half drains...I only get about 80 miles from the middle point of the gage to E...If I fill up a half empty tank, I'm getting close (not quite) 20 mpg...if I let it get near Empty I'm looking at about 14 mpg. I've checked several times and unless someone has a clue for me to investigate, I'll just keep filling up around half empty. : I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. : Itseems that I get about 140 iles to the first half tank...the second : half drains...I only get about 80 miles from the middle point of the : gage to E...If I fill up a half empty tank, I'm getting close (not : quite) 20 mpg...if I let it get near Empty I'm looking at about 14 : mpg. : I've checked several times and unless someone has a clue for me to : investigate, I'll just keep filling up around half empty. that's normal on all volvos i've owned. check it out. run to red... fill with 14.5 gals... at 1/2 point fill it... it will probably take 9+ gals... i've no good answer except don't believe the guage! it's the nature of all 245s i've owned... red face arguments when i claim that the tank is near empty and someone sees the guage at 1/2... use the odometer... one tank should get you 280 smiles... or something else is wrong.... relax, best js > I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. > Itseems that I get about 140 iles to the first half tank...the second > half drains...I only get about 80 miles from the middle point of the > gage to E...If I fill up a half empty tank, I'm getting close (not > quite) 20 mpg...if I let it get near Empty I'm looking at about 14 > mpg. Don't know but I noticed that in a 2000cc Nissan sedan I had. Never really checked other cars since (very often fill up at half empty anyway). That's measured by km & litres not the fuel gauge. Has to be to do with tank fumes or vacuum? > I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. > Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 5 lines] > I've checked several times and unless someone has a clue for me to > investigate, I'll just keep filling up around half empty. That is normal. The gas guage is an inexact indicator of the amount of gas left in the tank. I worked in the National Bureau of Standards (now National Institutes of Standards and Technology), 1957-1969. I realized that everyone in my car pool had mentally calibrated their gas gauge. When the guage first indicates empty, there is still a gallon or more of gas in it. That seems to be by design. > > I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. > > Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 13 lines] > gauge. When the guage first indicates empty, there is still > a gallon or more of gas in it. That seems to be by design. If Champ knows what the relative mpg's are, you would assume he's not basing that on the fuel gauge. I didn't when I found the same thing. I travel by interstate about fifty miles each day. In my S80 T6, when I drive the speed limit, the computer registers about 27 miles per gallon. On this same trip, when I drive 55 miles per hour, the computer registers usually over 34 miles per gallon. Tom and Ray on Car Talk says that the amount of air resistance is double for a car traveling 75 miles per hour compared to 55 miles per hour, so this gas savings makes sense. I think I will go slow. >> > I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. >> > Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 17 lines] > basing > that on the fuel gauge. I didn't when I found the same thing. > I travel by interstate about fifty miles each day. In my S80 T6, when I > drive the speed limit, the computer registers about 27 miles per gallon. On [quoted text clipped - 3 lines] > compared to 55 miles per hour, so this gas savings makes sense. I think I > will go slow. Think how much fuel consumption would drop if more people did that. Is there a big difference in your commute time when you lower your speed? If so, think of it as a way to appreciate more music. <g> >>>>I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. >>>>Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 16 lines] >>If Champ knows what the relative mpg's are, you would assume he's not >>basing that on the fuel gauge. I didn't when I found the same thing. >> I travel by interstate about fifty miles each day. In my S80 T6, when I >> drive the speed limit, the computer registers about 27 miles per gallon. On [quoted text clipped - 8 lines] > when you lower your speed? If so, think of it as a way to > appreciate more music. <g> Same observation here. The speed limit on I-10 in LA is 70 mph. I used to drive at 80 mph, and my fuel economy was about 25 mpg. I now drive 65 mph (in the right lane), and my economy is 32 mpg. AC  SignatureEmail: echo 142322093203359315271794620168064975321554275890186P | dc >>>I travel by interstate about fifty miles each day. In my S80 T6, when I >>>drive the speed limit, the computer registers about 27 miles per gallon. On [quoted text clipped - 14 lines] > > AC I don't subscribe to a conspiracy idea, but who benefits mostly from higher speed limits - the oil companies and the governments. Even with the low gas taxes that Americans pay, they are still paying a lot more than if they drove at the speed limit or if the speed limit was put back to 60 mph (even I wouldn't suggest 55). A gallon of gas burned today won't be available tomorrow, will it? If we won't think about the collective good, then think about our pocket books. > I don't subscribe to a conspiracy idea, but who benefits > mostly from higher speed limits - the oil companies and the > governments. . . . and drivers, who clearly want higher speed limits. Nobody is forcing drivers to drive > 55. That's what the vast majority of us want. Rick >> I travel by interstate about fifty miles each day. In my S80 T6, when >> I drive the speed limit, the computer registers about 27 miles per [quoted text clipped - 7 lines] > there a big difference in your commute time when you lower your speed? > If so, think of it as a way to appreciate more music. <g> Most people never bother to do the math... If you commute 30 miles and it takes an hour (not uncommon in LA traffic), you're averaging 30 miles per hour. Why then, is it necessary to go 70 mph, zero mph, then 70 again for a few minutes? Or, go 50 mph between signals, only to sit at the next red light? I only commute 10 miles. It takes between 20-25 minutes. I roll along at 30-35 mph while others go flying by at 45-50, only to sit at the next signal. Surprise, surprise, the light goes green just as I roll up to it and I never moved my foot on the gas or touched the gas wasters (brakes.) >>>I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. >>>Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 16 lines] > If Champ knows what the relative mpg's are, you would assume he's not basing > that on the fuel gauge. I didn't when I found the same thing. I've re-read his post to see if I missed something in my answer. He wrote in terms of miles per half tank of gas, as indicated by the gauge. If the gauge isn't linear, it doesn't translate accurately to MPG. I think my answer was correct. > >>>I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. > >>>Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 22 lines] > doesn't translate accurately to MPG. I think my answer was > correct. He wrote in terms of mpg, no more inference gals were measured by tank vol than by pump vol, except the gauge is used as a guide for when to fill up... as you do. While most gauges are inaccurate, most ppl don't use it to measure mpg. Your answer is correct in that most gauges bottom out before the tank does, but it's an assumption that is the reason for the mpg difference. I experienced the same consistent mystery in a Nissan... measured properly. Everyone assumed it was the gauge inaccuracy, but that was not the cause. >>>>>I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. >>>>>Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 34 lines] > measured properly. Everyone assumed it was the gauge inaccuracy, but that > was not the cause. He wrote: "Itseems that I get about 140 iles to the first half tank...the second half drains...I only get about 80 miles from the middle point of the gage to E..." That is equivalent to mpg only if you assume that each half of the gauge represents the same number of gallons. ........... > > He wrote in terms of mpg, no more inference gals were measured by tank vol > > than by pump vol, except the gauge is used as a guide for when to fill up... [quoted text clipped - 11 lines] > That is equivalent to mpg only if you assume that each half > of the gauge represents the same number of gallons. He also wrote "If I fill up a half empty tank, I'm getting close (not quite) 20 mpg...if I let it get near Empty I'm looking at about 14 mpg". That is (possibly) incorrect only if you assume it was based on the gauge. You think he didn't even twig that "near empty" could mean another 50 miles? > ........... > [quoted text clipped - 30 lines] > (possibly) incorrect only if you assume it was based on the gauge. You think > he didn't even twig that "near empty" could mean another 50 miles? If I could read minds, I'd know. My impression from reading between the lines of his question is that he assumed that his gas guage accurately reflects the volume of gas in the tank. My training as a scientist is not to trust any measuring device until I've calibrated it, unless it came from a source I knew did a good calibration. Sometimes, the exactness of the reading isn't vital. I know my bathroom scale is off from the scale in my doctor's office. But I use my bathroom scale mostly to tell me whether I'm gaining or losing weight, so it only needs to be consistent, not highly accurate. ............ > > He also wrote "If I fill up a half empty tank, I'm getting close (not quite) > > 20 mpg...if I let it get near Empty I'm looking at about 14 mpg". That is [quoted text clipped - 7 lines] > measuring device until I've calibrated it, unless it came > from a source I knew did a good calibration. ...can't read minds but can read between lines :) Sorry to be argumentive, when I arrived at the same conclusion as Marvin (measured properly) people assumed the fuel gauge was to blame. A lesson in how quick many are to assume one is stupid, I give more credit for intelligence as a direct result. > ............ > [quoted text clipped - 24 lines] > assume one is stupid, I give more credit for intelligence as a direct > result. Every fuel gauge I've had a chance to check for lineraity failed the test. Others report the same experience. It is a reasonable and likely explanation of the original poster's observation. ........... > Every fuel gauge I've had a chance to check for lineraity > failed the test. Others report the same experience. It is > a reasonable and likely explanation of the original poster's > observation. You only have to look at the arrangement to know it won't be very accurate, it's an equally big assumption the inaccurate gauge was used to arrive at firm mpg numbers. Since I have repeatedly varified the same behaviour in another car, measured by the pump, for me the obvious explanation is much less likely. Rather than dismiss it based on assumption, I am interested in why it happens at least in some cars, because I know it does happen. Well...I have done the first two fill-up calculations....and there is nothing that dispels my belief that I get better mileage early in the tank. I filled up at a half tank...I got 163 miles on 9.18 gallons of gas I then filled up at around 3/4 tank....the gage only has a hash mark at the half tank and the red line at about an 1/8 tank... the 1/4 tank was actually 5.25 gallons and I got 104 miles on that 5 gallons. So, those who said the gage is not calibrated to precision are correct, but I still seem to get better mileage with more fuel in the tank...go figure. Mike >I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. >Itseems that I get about 140 iles to the first half tank...the second >half drains...I only get about 80 miles from the middle point of the >gage to E...If I fill up a half empty tank, I'm getting close (not >quite) 20 mpg...if I let it get near Empty I'm looking at about 14 >mpg. >I've checked several times and unless someone has a clue for me to >investigate, I'll just keep filling up around half empty. Gas gauges are not precise. The only way you can come close to calibrating it is by doing repeated fillups; starting with a full tank, write down the mileage (don't depend upon your memory), then: Run down to 3/4, fill up, write down mileage; Repeat twice; Run down to 1/2, fill up, write down mileage; Repeat twice; Run down to 1/4, fill up, write down mileage; Repeat twice. Now calculate the mileage in each case, based upon the exact miles driven and exact fuel consumed. I think you'll find it's nearly the same in each case, and determine that 1/2 on the gauge doesn't equal the tank at 1/2 capacity. Gary SignatureGary Heston gheston@hiwaay.net I don't need an iPod, I have an IQ. A worthwhile endeavour: http://www.thebrestcancersite.com/cgi-bin/WebObjects/CTDSites I have checked at most of my fill-ups...resetting the trip odometer and calculating the mileage one the precise amount of gas based on the reciept. I have found that the first half gets better gas mileage than the second half. I thought perhaps something with the pressure was to blame. I will undertake this as a true science project and starting with my next fill-up (soon considering the crappy mileage I'm getting) and determine the gage's accuracy and the mileage for the various scenarios. Thanks, Mike >I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. > Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 5 lines] > I've checked several times and unless someone has a clue for me to > investigate, I'll just keep filling up around half empty. The gauge is non linear... basically, the bottom half of the tank is typically smaller on most vehicles because they are either tapered, or have a twin tank setup (like the 760) or some other odd configuration that makes the bottom half of the tank actually hold less fuel. The sender doesn't care how much fuel is in the tank, it just knows where the float is (relative to an odd shaped tank). Almost all vehicles will reflect this, and this is normal. I get around 21 mpg BEST or 18.5 worst from my 760GLE with a B280F 6, so I would expect you to get around the same with the turbo... I'd get worse, cuz I'd be always on the turbo :) Cheers ............ > The gauge is non linear... basically, the bottom half of the tank is > typically smaller on most vehicles because they are either tapered, or have [quoted text clipped - 9 lines] > would expect you to get around the same with the turbo... I'd get worse, cuz > I'd be always on the turbo :) And the radial swing on most float arms would affect readings too, but the tank could be shaped like a funnel if comsumption is measured properly, which it sounds to me it is. Fill up after 200km and then after 400km - see if it takes exactly double the fuel. I once heard a talk by a guy who was a gas gauge engineer for Cadillac (yes, there is such a job). He explained that with modern microprocessor-controlled digital instrumentation they could make the gas gauge perfectly linear, or whatever curve they choose. As he told it, the profile was defined by their marketing group and was deliberately not linear. As I recall, they had designed it to be very slow at the beginning (so people feel good about their recent fill up), quick in the middle, and slow again at the end (so that if you're running low you get a little extra chance to get to a gas station). I'm not sure about Volvo, but I think most manufacturers have gone away from the float ball to some sort of linear submersion sensor with no moving parts. Signature----- -RL >>I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. >> Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 21 lines] > > Cheers > I once heard a talk by a guy who was a gas gauge engineer for Cadillac (yes, > there is such a job). He explained that with modern [quoted text clipped - 9 lines] > the float ball to some sort of linear submersion sensor with no moving > parts. That would be a weight gauge, which indicates the depth of the gasoline layer in the tank. It would be affected by the shape of the tank. > I once heard a talk by a guy who was a gas gauge engineer for Cadillac (yes, > there is such a job). He explained that with modern [quoted text clipped - 5 lines] > slow again at the end (so that if you're running low you get a little extra > chance to get to a gas station). Wonder what other bullshit features were in that car... exaggerated body roll to make you think it went around corners faster? It would be taking extreme liberties to deliberately make a gauge inaccurate, though they never are perfect. It's also a big presumption that a mech/elec gauge could not be made reasonably accurate or its behaviour could not be manipulated. If you didn't fill the car & note the miles, the gauge is all you have to go on - if it wasn't reasonably accurate, especially if by design, it wouldn't impress me at all. >> I once heard a talk by a guy who was a gas gauge engineer for Cadillac > (yes, [quoted text clipped - 21 lines] > if it wasn't reasonably accurate, especially if by design, it wouldn't > impress me at all. what really surprises me is that Volvo never included a LOW LEVEL warning light... my 70's Fiat had that, my '86 Corolla had that... but my 760 doesn't!! My 760 ('87) also doesn't have tilt steering, while my Alfa Romeo of the same era had tilt/telescope!! What's up with that?!?! Of course Volvos have a low fuel contents light. It is part of the fuel gauge and is a small Red or Orange dot that lights up when there are about 6 Litres of fuel left in the tank. Your 760 is just before the facelift model which does have the steering wheel tilt as do subsequent 760s and 960/S90/V90s. The tilt lever is to the left of the wheel (UK) between the casing and the dash knee roll. Might be to the right on USA/Canada models. All the best, Peter. 700/900/90 Register Keeper, Volvo Owners Club (UK). >>> I once heard a talk by a guy who was a gas gauge engineer for Cadillac >> (yes, [quoted text clipped - 31 lines] > 760 ('87) also doesn't have tilt steering, while my Alfa Romeo of the same > era had tilt/telescope!! What's up with that?!?! .......... > what really surprises me is that Volvo never included a LOW LEVEL warning > light... > > my 70's Fiat had that, my '86 Corolla had that... but my 760 doesn't!! My > 760 ('87) also doesn't have tilt steering, while my Alfa Romeo of the same > era had tilt/telescope!! What's up with that?!?! People are often surprised when they discover what has been traded for economy, convenience, sales attractiveness or someone's idea of progress. Just think if it was a Ford device you'd probably be better off without it. > .......... > [quoted text clipped - 8 lines] > economy, convenience, sales attractiveness or someone's idea of progress. > Just think if it was a Ford device you'd probably be better off without it. If you wen't back as far as I do, you would remember when the dashboard had a meter that showed how much current was going out of or into the battery. Voltage regulators weren't so good, and we worried about overcharging the battery. But we didn't have an outside temperature reading. >> .......... >> [quoted text clipped - 17 lines] > battery. Voltage regulators weren't so good, and we worried about > overcharging the battery. ...and if you didn't flash the field on the generator when you changed the battery, it wouldn't charge at all. > .......... >> what really surprises me is that Volvo never included a LOW LEVEL warning [quoted text clipped - 10 lines] > Just think if it was a Ford device you'd probably be better off without > it. hehe... well, I think a safe thing would be a low level light, prevents you from getting stranded on the highway. Another poster suggested it comes on at 6litres... I've never been that low, but that is QUITE low, the gauge dips way into the red and I start to sweat there.. the light has never come on since the car was new.(where is it? it's not a separate indicator which is obvious, unless it is backlit in the red section, which I doubt... I can take a pic). Oh, and Fiat 500s didn't have a fuel gauge, JUST a light ;).. > I have a 1990 765 Ti...I get bad gas mileage...but wait there's more. > Itseems that I get about 140 iles to the first half tank...the second [quoted text clipped - 5 lines] > I've checked several times and unless someone has a clue for me to > investigate, I'll just keep filling up around half empty. If you look at the profile of a fabricated steel fuel tank they are almost always narrow at the top fat in the middle and narrow at the bottom. So from full to half the distance per volume measure is decreasing and from half to empty the distance per volume measure is increasing. The variable resistor that is the fuel sender is usually linear wound so that it really represents distance travelled through its sweep rather than volume of fuel consumed. Kind or like an electronic dipstick. The blow molded plastic tanks are more boxlike but because they can be easily formed to accomodate the available space they can have even moew radical volume differences on cross section. Bob SignatureThe goal when driving is to miss the maximum number of objects. |
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