Puzzing Gas Problem?

[Kenrik]

Ok so I filled the tank on my G35 Coupe (17.5 Gal is what I pumped)I drive 298 (almost all Highway 80 - 90 MPH) and it's one notch above empty.. where the yellow light comes on, (18.625 MPG)

I fill her up with 8 gallons, and I drive 200 miles (80 - 90 MPH) untill it's back down to that same SPOT! (23.5 MPG HWY)

???? WTF ?????

I have noticed that since I got the car I always get better gas mileage when I put in less gas.

6 Gal ALWAYS = 120+ miles (city highway mixed, 20mpg)

But the MOST I ever got out of a FULL TANK was 330 Miles (18.8 MPG)

I have no idea why it uses more gas when the tank is full?
Modified by Kenrik at 8:38 AM 8/14/2007

[joe603]

Weight?....one gallon of gas weighs about 6.5lbs, so if you don't put in 9 gallons, that's just under 60lbs of reduced weight, which continues to drop as you use more fuel.

[Gimli G35]

weight adds on to the needed power to move the car .. just like when you remove the back seats out of a SUV , you will get a few more miles

[Kenrik]

^ I had thought about that, and it make sence EXCEPT... I had my future mother in law (maybe 200lbs) and my Future wife (95 LBS) in the car with me for both fillups... that's a lot more weight and still I got 18.6 MPG on the full tank and 23.75 MPG on the 8 gallons.

[Gimli G35]

i have no idea now . im still saying it weight it could be ... types of road conditions , what you were using in the car ( A\C and all that ). thats all i can think of that might cause that problem..

[ArizonaG35]

Hey Ken,

I've noticed a similar (However different) issue on my G as well.... When I first FILL up the tank to the top and reset my MPG gauge, it reads 28MPG for the first 30-50 miles, then gradually drops down to 20-21 MPG by the end of the tank... I'm getting 360-385 miles per tank of premium...

You should know that the MPG reading with the on board computer is not a truly accurate gauge of your miles per gallon usage. The most accurate way to determine your MPG is to fill up the tank (from empty), then reset your trip meter... run down the tank til it's on fumes, then divide the number of miles you drove by the number of gallons you put in the tank... this will give you an exact MPG reading...

EXAMPLE:

18 gallons of fuel385 miles driven

385 Divided by 18 = 21.39 MPG

[Kenrik]

i have no idea now . im still saying it weight it could be ... types of road conditions , what you were using in the car ( A\C and all that ). thats all i can think of that might cause that problem..

it always does it though, every time I fill the tank all the way up the gas mileage is worse?it makes no sence at all, I have even tried to fill it up and drive like a pansy to save gas and the mileage still sucks...

Then I put in 3 gallons on an empty tank, shifting at 4-6k and chirping my 275 tires and I get 60 miles (20mpg) WTF?
Modified by Kenrik at 10:28 AM 8/14/2007

[Kenrik]

Hey Ken,

I've noticed a similar (However different) issue on my G as well.... When I first FILL up the tank to the top and reset my MPG gauge, it reads 28MPG for the first 30-50 miles, then gradually drops down to 20-21 MPG by the end of the tank... I'm getting 360-385 miles per tank of premium...

You should know that the MPG reading with the on board computer is not a truly accurate gauge of your miles per gallon usage. The most accurate way to determine your MPG is to fill up the tank (from empty), then reset your trip meter... run down the tank til it's on fumes, then divide the number of miles you drove by the number of gallons you put in the tank... this will give you an exact MPG reading...

EXAMPLE:

18 gallons of fuel385 miles driven

385 Divided by 18 = 21.39 MPG

I don't have Navi... The method I use is the one you just pointed out...

[telcoman]

I never gave weight of gas a thought but I believe that temp and humidity also are a factor? I usually fill up in the early morning hours when leaving for work when it is still cool outside. If you fill up during the hottest part of the day I believe as temp goes down in the evening the amount of gas in the tank will shrink? I've always used regular 87 octane usually BP but also Raceway, Pittstop and Hess. Whatever is cheapest. After the pump clicks off I may add another 1/2 gal to round off the price. Over time this will even out. I divide total miles traveled by number of gallons to fill the tank.I have a cheap calculator in the glove box.My best mileage on my 06 G35 Sedan 6 sp manual was 28.83 on a tank on a trip to Florida. The entire tank was on cruise control with A/C on.I drive over 100mi/day to work & my mileage here in NJ generally varies between 23.5 and 25.5. My mileage has also improved as total miles increased. I now have 36.4k. Also my oil consumption improved & went from 1000 mi/quart when new to over 2000 mi per quart now.

Telcoman

[Gimli G35]

You have a weird problem, and if what Telcoman is saying IS true ... holy crap .. i will have to test this out I dont know if gas shrinking is true though.

[ArizonaG35]

I cannot confirm the information contained in this site, but these people do have a lot of fancy graphics and charts that looks pretty! This web page has been distributed around Arizona for a while now, and I believe it to be true that Gas does expand and that you don't get all you pay for if you buy "HOT" gas...

Just food for thought... I think this would be another good candidate for mythbusters!

http://www.users.qwest.net/~taaaz/AZgas.html