A gas cap, the plastic or metal cap that secures the entryway into a vehicle's gas tank, is secured in place using a screw-style fitting. To be sure the gas cap is held in place without danger of falling out or becoming loose, most gas caps employ metal or plastic threads that match up to a groove in the fuel tank piping; this thread and groove allow the gas cap to form a secure seal much like a threaded screw holds securely in a wall. Depending on the specific make and model of the vehicle, the gas cap may have enough thread for two or three full rotations when being inserted into the fuel tank piping. Some gas caps also end the threading with a flat series of plastic notches so that the gas cap can be turned with moderate resistance when it has been fully inserted.
Gasoline, especially higher octane fuels, tends to vaporize and evaporate readily if exposed to the atmosphere. When securely in place, a gas cap helps keep the gas tank under pressure and reduce the evaporation process; by doing so, the gas cap helps prevent fuel vapors from escaping the gas tank and venting into the atmosphere. This precaution greatly enhances gas mileage, as evaporated gas cannot be effectively used as fuel.
In addition to being detrimental to the effective gas mileage of the vehicle, gasoline fumes can present both environmental pollution concerns and a safety hazard to vehicle occupants. A loose gas cap can create fuel vapors that, when released into the atmosphere, pose a serious danger to plant and animal health in addition to concentrated flammable vapors that can be readily ignited. Finally, occupants of a vehicle who inhale large amounts of vaporized fuel may experience headaches, dizziness, or--in extreme cases--death. According to automotive experts at the National Association of Stock Car Auto Racing (NASCAR), as much as 147 million gallons of gasoline vaporize each year due to faulty gas caps, so a properly functioning gas cap is a critical component of an automobile.
