jaguargod

Does anyone else experience this? I get pretty fair fuel mileage. About 270 miles on about 14.5 gallons. But everytime that I fill up, after I drive about 25 miles, the first quarter tank is half gone. The second quarter tank lasts a little longer, and obviously I begin to get much better fuel economy on the last half tank. Does anyone know why the car would go through the fuel so much more quickly on the first two quarter tanks than the last two? If they would burn as slowly as the last two quarters, I would be breaking 300 miles per tank easily. Is this a true measurement of fuel consumption, or is the gauge not all that accurate?

Glyphon

just guessing, but it could have something to do with the shape of the tank. like if it was tapered so that it was larger at the bottom, then it would appear to use gas faster in the top 1/2 (according to the guage) than at the bottom.

Go48

Fuel gauges are not precision instruments and they are notoriously non-linear. The purpose of the gauge is to provide the driver with a general idea of fuel status, it's not designed for making accurate, on-the-fly calculations of fuel usage/mileage.

Razpewton

Try placing 8 one-gallon jugs of water in your trunk, and see what happens to your gas mileage....

mikeb

I get the same thing going on

OICU812

it has to be the shape of the gas tank .my truck does just the oppisite .most tanks are small on the bottom and big on the top that why it takes $15 dollars to reach a 1/2 of tank but cost you 25 more to fill it up .

zitch

Most sources place gasoline of being 5.8 - 6.5 lbs per gallon. So 8 gallons would be 46.4 to 52.0 lbs. Only about 2% of the weight of the car. I doubt that would affect your gas mileage any noticeable amount...
Some facts and theories as to the fuel gauge seems to differ from the first half to the last half:

1) Most fuel tanks are not a standard shape. Most are shaped funny to be able to fit in the car with the most available internal volume.

2) Most fuel guages use a float to tell the level of the gas. When the tank is full, the float is often submerged in the gas and won't move until some gas is used.

3) Some car designers purposefully design the fuel guage to act in a certain manner that would make sense. Think about it, are you more interested in the guage when your tank is one gallon short of full, or when it has one gallon left?

wintergrn

"Linear fuel gauge" is one of those terms like "jumbo shrimp." Ours is particularly sensitive near the top end -- I swear mine drops below F as I'm pulling away from the pump.

Come to think of it, our oil dipstick has a nonlinear scale too.

Nubo

Yep, mine drops alarmingly from full, and gradually "slows" as it moves towards empty. It's non-linear, and probably by design. Iirc, the behavior of the needle seemed to change when the newer flashes came out (going from K to M). I don't think the shape of the tank has much to do with it since the gauges are controlled by the ECU, it could easily calibrate the float position to true volume. Probably a concious decision that drivers are more comfortable with the gas seeming to stretch the last half or quarter tank. Or perhaps a sense of greater accuracy (in terms of how close to empty) if the low end is stretched out. But seeing the needle move a notch off of Full just going around the block is disheartening. Not a big deal; the human brain calibrates fairly well. Would be a pain though, if a future flash re-calibrates the gauge the opposite way.

rotten42

Mine's the same way...I go 3 feet and my car syas a quarter tank is gone.....the bottom half of the tanks last much longer.

Johnnyma45

I consistently get 50 miles until the first quarter notch, 100 by the half, 200 by the 3/4, and 30 the rest of the way. I've gotten maybe 270 one time...and that was scraping the bottom. I dunno...the 8's gas guage is the most fluctuating one I've ever driven. Other cars were more consistent/accurate.

jaguargod

I don't think that the gauge is accurate near full or empty. It drops horribly fast at first, and then after it drops below the "E", I drive another 50 miles sweating that I am going to run out of gas, and then it only takes 14.5 gallons, which means I still had about 1.5 gallons left.

lucidz

i think the best way to measure is to just keep track of the miles and how many gallons you put in, so you have an exact number.

i also refuse to even drive if i dont have at least 1/4 of a tank. I always get gas first.

PS. I've been getting about 14mpg around town

Glyphon

thats a good way to figure out what your fuel economy is...its a poor way to tell how much gas you've got left in the tank.

and as for your 14mpg, you are either in your break-in period, or you need to use less throttle/keep it under 3750 more. I have 1 redline (minimum) per day, and have no problems getting into the 17-18mpg range with ~90% city driving.

lucidz

i've got around 17K miles on the thing actually, so should be way past break in. Past that, I keep it at 2-3.5K rpms almost all the time, i never hardly get on it

MazdaManiac

iTrader: (3)

Interesting. Quite a lot of variation.

I get exactly 50 miles on each and every quarter on the gauge with an additional 17 miles or so after the empty line.
The light comes on about 10 miles before the empty line. This is highly consistent.

Blue87Sport

I used to get almost exactly 40 miles on the first quarter on the gauge consistently. Since I got the N flash (about 2 full tanks), it is more like 50. Total miles per tank are about the same. Does the ECU programming manage the gas gauge???

Go48

The instrument cluster has it's own microcomputer to control the gauges, warning lights, various other information displays, etc. Data from the fuel gauge sending unit in the tank is sent directly to that microcomputer, so I don't think the PCM is involved in the determination and display of fuel levels. At least that's my interpretation of the wiring diagram.

Microdot

You've just described my 8's fuel consumption habits exactly.

I know I could get Slightly better milage by keeping the average RPM's a bit lower. ...but that isn't anywhere near as much fun. ...and it doesn't seem to help by all that much, so why bother. I bought this car to drive and enjoy. I've just come to accept the fact that it get's thirsty when I'm having my fun. :D

Razpewton

I know it won't have any bearing on your mileage. I was just seeing if anyone would drive around with 8 gallons of water in their trunk. :D :D :D

fredw1

Consistent 50 miles for the 1st quarter, 100 miles for half. I try to fill up before three quarters so I don't much data for the second half. Since my mpg is close to 17 each tank (that would mean 6 gallons for the first half), the second half would likely be a little better.

RX8-CARL

I've asked this before and nobody answered.
I think the gauge has been reprogramed to make you think you are getting better economy. I'm getting slightly better economy since flashed, but nowhere near what it appear's on the gauge.
At most 1 mpg but 15 - 20 miles more per 1/4.

Go48

Answer: Your supposition is probably incorrect. Read my previous posting in this thread (#18). Because of the use of a separate microcomputer to manage gauge display in the instrument panel, it is unlikely that a re-flash of the PCM would have an effect on fuel gauge reading. A PCM re-flash could have an effect on real fuel mileage as you suggest, but doubtful that the re-flash would re-calibrate the fuel gauge and/or fuel level sending unit.

Up on soapbox:
One more time. Fuel gauges are non-linear and notoriously inaccurate. So why are people attempting to use them to determine fuel mileage? It's an interesting academic exercise, but it is meaningless when actual fuel mileage--not a reading on a fuel gauge--is what one needs to know. Try it: Miles traveled divided by gallons added to fill tank=actual fuel mileage. Easy.
Down from soapbox:

RX8-CARL

I've Been keeping a spreadsheet on fuel economy for year's, Milage & actual fuel used.
Thats why I know the gauge has changed but the MPG is almost the same.
If a separate microcomputer is use then could this have been reset.