[givemeslack]

I agree with 2000neon, that the downside to only filling half a tank means not easily getting accurate gas mileage that you would by measuring miles from fill up to fill up.

Fortunately one of my cars, a Maserati Coupe, has a mpg gauge. I'd much rather fill 5 gallons at a time for pleasure trips, than the full tank of 22 gallons in that vehicle, especially since I don't use it everyday, and gas does go stale. Driving it with only 5 gallons and less is noticeably faster, too. There's a reason why cars are stripped down for track racing.

Even without a mpg gauge, you can still use my method of only filling up halfway AND get an accurate reading. Start with a full tank, setting the trip odometer to 0. Drive and use up the gas almost to empty.

From then on, whenever the tank is empty, only fill it up half way, keeping a log of the gallons of gas put in, (or just save all the receipts). Do this as many times as you want, the more the better, and when you are ready to take accurate reading of mpg, fill the tank to the top again. Figure the mpg by the odometer reading, and all the gas added since the last complete fill up. Simple.

You can't deny that it's significantly less weight to drive with half a tank of gas or less, than with a full tank of gas, and this must increase the mpg. Gasoline weighs about 6 pounds per gallon. In a 22 gallon tank, that's like carrying an extra person.

And, so, I strongly disagree with Gealii. Pulling in to a gas station that is on your route, shutting off the engine, and filling up with however many gallons you want does not waste gas. It only takes a few minutes, and it's a good time to pull over and check the oil and tire pressure.

Either way you do it, it does make sense for several reasons to let the needle go all the way to 'E' before filling up.