Gas mileage mystery

ChampaignTurbo wrote: : 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

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?

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.

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.

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

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

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 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.

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

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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.

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.

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.

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.

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

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 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.

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.

. . . 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