why does the gas pump say "do not top off"?

Answer 1

http://www.epa.gov/donttopoff/

Answer 2

Several things. First: gasoline vapor emissions. Topping off usually ends up spilling fuel, the vapors are pure hydrocarbons. Second: Topping off leaves liquid fuel in the vapor recovery hose at the dispenser. Liquid in the vapor line causes a valve to close at the end of the hose preventing liquid fuel from getting into the vapor return system of the gas station. The nozzle will keep shutting off because the vapor return is closed. The hose is actually two hoses, vapor in the outer section and liquid through the inner hose.
Third: Fire hazard, again spillage is more likely to occur while topping off.

Answer 3

That means when the pump stops itself the tank is full, but most people keep squeezing a little at a time until they hit the next whole dollar. the problem is that sometimes there is less room in the tank than you realize and then you end up with a gas leak.

Answer 4

Do Not Top Off

Answer 5

Modern automobiles have a fuel tank purge canister that purges fuel vapors out of the tank. If it becomes contaminated with raw gasoline it will ruin it and it is quite expensive to replace. If you top the tank off the chances of that happening are greatly increased... http://www.jpsalestraining.com/topping_off.htm

Answer 6

By topping off tank possibly the vapors may explode when you turn over ignition? also your gas has nowhere to expand when heated up..

Answer 7

That means not to tip your hat while filling your tank as the static created between your hair and the hat can cause a spark to jump from your hand to the nozzle. If that happens, you will look like a dork to the young lady you were doffing your hat to, because now your clothes are on fire and you are hopping around and "Oh! It burns!"