When you open the fuel bay door on any vehicle, the gas cap is readily apparent. It is the physical seal that keeps gasoline inside your car's tank and the environment out. The gas cap performs three functions: improving safety, increasing fuel economy and reducing emissions.
Gasoline is highly volatile, meaning liquid gasoline vaporizes into a gas readily at room temperature. Since gas vaporizes easily, if your tank was not closed by a tight cap, highly flammable gasoline vapor would constantly leak from your tank.
Gasoline normally burns cleanly within a vehicle's engine. If gasoline vapor was to escape without being burned, it would damage the atmosphere like many other volatile hydrocarbons. The gas cap prevents leakage of atmosphere-depleting vapors.
For every gallon of gasoline vapor that escapes from an open tank, a gallon of usable fuel is wasted. Gas caps make sure the gas you pay for at the pump actually reaches your engine.
