When you are filling up your gas tank, do not put the gas in at the fastest nozzle setting. If you look at the nozzle, you'll see that they usually have three settings: low, middle, and high. If you use the slowest setting, it will minimize the vapors that are created while you are pumping your gas. This will in turn maximize the amount of gas that actually ends up in your tank. Gas pumps have a vapor return mechanism. If you are pumping at the fastest rate, more of the liquid gas will be converted to vapor and that will be returned up and back into the underground storage tank so you'll be getting less gas for your money.