Is it bad to overfill your gastank?

Answer 1

Yes. Five percent of your gas tank's volume is reserved, and required, to allow for gasoline vapors to expand. So if you have a 25 gallon tank, the the most it will hold is 23.75 gallons. Overfilling the tank can cause vapor lock in the fuel line, damage to the fuel pump, your car's PCM to throw false trouble codes, federally required emissions tests to not be run, and in worst cases, the car to catch fire (due to excess vapors around the tank and/or raw gasoline dripping from the vapor vent).