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The dangers of overfilling your gas tank

You get a few more miles by topping off your gas tank, but it's not worth the risk.

American Petroleum Institute

The temptation to keep clicking the gas station pump is strong, but it's probably not worth it. The potential damage to your car, your shoes and the environment are significant.

Here's why "topping off" is probably best avoided: 

You might damage your car: All US market cars sold since 1971 have a closed circuit system to trap and later burn the fumes from their gas tanks. A complex array of charcoal canisters, tubes, valves and sensors comprise a system designed to handle gasoline vapors only, not liquid fuel. Jamming too much fuel into it repeatedly can result in a Check Engine state that will keep your car from passing an emissions test and could cost hundreds of dollars or more to repair.  

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The vapor recovery system in a modern US market car has evolved to an elaborate (and expensive) sub-system. The charcoal canister in particular does not take kindly to being overwhelmed with liquid fuel, which can result from regular overfilling.

Bosch

You'll waste money: When you force-fill, excess either flows out into a little drain below your fuel filler neck and onto your shoes, or it flows back into the gas pump in which case you're paying the gas station to take some of its own gas back.

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You release emissions: Overfilling can defeat the vapor recovery nozzles found on many pumps.  It's not a huge release of fumes, but why do it? After all, you'll breathe it first: You're the one standing right there. 

It seems tempting to ram more fuel into your tank to delay the next fill, but I've found my cars only take another fraction of a gallon when I do it, rounding off to about 15 more miles in real world driving. Stick to one click.