Gas Milage Question

My wife tells me cars get better gas mileage when the tank is at least 1/2 full. She claims it has to do with pressure in the tank. I maintain the amount of fuel in the tank wouldn’t have much to do with it compared to driving habits, etc. and possibly the less fuel in the tank, the better gas mileage (less weight of the car). Please help us solve this question before things get really ugly! :smack:

I don’t have a definite answer, but I have noticed that in most cars I have driven the gas gauge seems to either speed up or slow down after the 1/2 point. I think this has more to do with the shape of the tank or the accuracy of the gauge rather than the milage of the car.

My Ford Explorer gives a digital readout of the MPG. This can be done instantly or shown as a rolling average. I have never noticed any difference in the bottom half of the tank.

The latter. See HowStuffWorks, and also here and here in the archives.

Err, I meant the former. I.e. “more to do with the shape of the tank or the accuracy of the gauge”. Honest, I did.

You’ll actually get better gas milage when the tank is less than half full, since you’re lugging around less weight. The “pressure in the gas tank” theory doesn’t make much sense, since the fuel pump takes care of that.

If more gas in the tank meant more fuel pressure–and it doesn’t–it would result in higher fuel consumption and poorer mileage.

In carbureted cars, too high a pressure is basically unheard of, a non-problem (in the lines–an overfull carburetor is a different thing). Too low pressure can result in fuel starvation, which may save mileage 'cause you can’t move very well.

In fuel injected cars, fuel pressure is regulated by a pressure regulator, and is constant regardless of fuel tank level (until you run out).

I don’t know where the idea of a fuller tank giving better mileage came from, but I haven’t run across any mention of it in 30 years of auto repair. As mentioned above, the fuel gauge is NOT a precision instrument.

It’s just an excuse to get you to leave her with some gas in the car