Totally Strange prob. very different MPG for the 1st 6 gallons and last 6 gallons
 

Background. 2005 AT, basic. 70k miles on the car, recalled engine with 5k miles.

I know the reading on gas gauge is not linear. I filled my 8 anytime when the gas price drops, no matter the tank was empty, half full, 1/3 full, etc. After many times, I know exactly on the gas gauge which place indicates 6 gallons consumption, and where is 12 gallons consumption.

For long, I've been suspecting that different MPG for the 1st 6 gallons and 2nd 6 gallons. I did two tests these weeks. It turned out to be a HUGE difference.

What I get(2 tests are basically the same):

1st 6 Gallons all highway MPG(from full tank of gas to 6 gallon mark): 17 (with cruise at around 70MPH)

2nd 6 gallons all highway MPG(from 6 gallon mark to 12 gallon mark): 23-24(with cruise at around 70MPH)

========

I did two tests, each time when the arrow reached 6 gallon mark, recorded MPG; then when the mark reached 12 gallon mark recorded again.

Sure there are some errors, like the weather, temperature, wind, etc. But personally dont think those can make such a big difference. I know 2 tests may not be enough, but I feel this thing for long, everytime the 1st 6 gallons of gas just disappeared in a light speed and the last few gallons in the tank seems can stay forever. Tho- taken together, the MPG per tank is still reasonable.

Does any one actually see the similar problem like this? How do you think?

Does that mean I should also keep my tank half empty?? LOL

Its cause you have a blue rx8, get a BB rx8 and u wont have that problem.

Try adding ballast to the rear seat equivalent to 6 gallons of fuel and repeating your experiment with half a tank. If you get the same numbers as you saw with a full tank then it's due to the weight of the fuel, and might indicate why Mazda chose the size fuel tank they did, given the mileage that the engine is able to return.

If this is done at cruising speed (with minimal acceleration), then weight isn't going to be a factor.

sz, if you are only using the fuel gauge and odometer/tripometer to measure MPG, there's no way you are getting an accurate comparison. I understand that you think you know where six gallons is on the gauge, but I just don't buy that your eyes can read that scale accurate enough. I mean, even if you are off by just one gallon, that can equate to a few MPG in the calculations.

You'll have to find a more reliable and repeatable methodology. The most reliable method is to start with a full tank and zeroed tripometer. Drive how many miles you want, fill it up, and divide the gallons into the miles driven. I would suggest doing this for what you deem as the first six gallons, and then do it again for a total of what you deem to be 12 gallons... And then compare those two results.

Get a ScanGauge. Then you'll know exactly what your fuel consumption is at all times.

^^ this.

You are correct, gas gauges are highly non linear. In fact, many manufacturers purposely mis-calibrate so that the FULL to 1/2 range decreases slower than the 1/2 to EMPTY range. This is done so that you don't see the needle drop so quickly right after you fill up the tank. It's supposed to make you feel good about your car: it's not burning up that expensive tank gas as quickly as it really is. No kidding.

This appears to be what you are observing. I haven't noticed it so much in my RX-8, since I almost always fill from 1/8 to Full, but it is very obvious in my Dodge truck where the needle barely moves for the first 100 miles. Once it's below 1/2 it drops like a rock. And their programming of the gauge works on me - I'm happy to see the gauge above 1/2 a tank for 3/4's of the time, LOL...

It could be due to the decreased density of the fuel without the wieght of the fuel above it, or decreased pump capacity due to lack of head at the pump suction...








...But I doubt it.:wiggle:

Yup.

what i do to see the crazy inconsistencies in the gauge is leave the key in accessory mode and fill up and just watch the gauge since the pump pumps pretty consistently you'll see the gauge shoot up at parts and others just not move at all (typically right below half it will sit in one spot for almost 2 1/2 gallons.