Today: Tom and Ray settle The Great Gas Tank Debate!

Dear Car Talk

Dear Car Talk | Jan 01, 2008

Dear Tom and Ray:

I am having a debate with co-workers about whether filling your gas tank halfway is more or less efficient than filling your gas tank completely. What is the most efficient way to fill a tank? Does the weight of a full tank affect the mileage? Does gas evaporate more quickly when the tank is half-full? I need your help to decide this debate and to decide the most efficient way to fill a gas tank. Thank you! -- Ian

RAY: I take it you're worried about the weight of the gasoline, Ian. You think that a full tank of gas weighs more, and therefore reduces your mileage. And by filling up only halfway, you'll have a lighter car and get more miles per gallon.

TOM: The answer is, that's true. I mean, sure, theoretically, if your 3,000-pound car is 50 or 60 pounds lighter, you'll get slightly better mileage. In fact, manufacturers spend all kinds of money taking ounces of weight out of various car parts.

RAY: Right. And there's no fuel lost to evaporation, because the fuel system in a modern car is a closed system.

TOM: But if you fill your tank only halfway, there's another major fuel loss. That's the fuel you use driving around looking for a gas station twice as often. And that could easily wipe out any small savings you get from making your car lighter.

RAY: Plus, where do you stop? If you're really determined to reduce the weight of your vehicle, shouldn't you always relieve yourself before driving, because otherwise you'll be carrying around THAT extra weight, too?

TOM: And shouldn't you shave your head to remove the weight of your hair? And your chest and legs, too?
RAY: And don't forget your back!

TOM: Then you'd also need to always drive nude. Especially in the winter, when those heavy woolen overcoats are murder on your mpg, Ian. Think about it.

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