- Up until the early 1960s, gas caps for automobiles were crude devices constructed of metal and were used only to prevent gasoline from splashing out of the vehicle's gas tank. The most sophisticated gas caps at that time were equipped with vent holes that equalized the air pressure inside and outside of the gas tank. When Congress started passing vehicle safety and emissions legislation in the late 1960s and '70s, vehicle manufacturers began developing more advanced gas caps that would prevent gas leakage during car accidents, such as rollovers, decreasing the risk of fire. They also created more environmentally friendly gas caps as previous designs allowed hydrocarbon gases to leak into the atmosphere, which contributed to ozone deterioration. This also spurred the development of other vehicle technologies for monitoring emissions.
- The main functions of gas caps today are enhancing safety, fuel economy and environmental soundness. All gas caps are designed to prevent fuel spillage during collisions, reducing occupants' risk of injury due to fire. They also prevent vapors from leaking out of the gas tank that would otherwise create a constant fire hazard and would pollute the atmosphere, causing damage to the ozone. Proper use of the gas cap also increases fuel economy by effectively eliminating gas evaporation.
- The most common type of gas cap is the fender well type. Installed in most new vehicles, fender well-style gas caps are made of zinc and are not designed to be exposed. Exposed gas caps function in the same manner as the fender well gas cap, but are designed to be exposed without a door. These gas caps are usually made of chromium and nickel, as opposed to a zinc covering, and blend into the car's overall body style.
- Gas caps, particularly aftermarket gas caps, offer several features that enhance safety. Gas caps with the "Pre-Vent" feature are designed to prevent gasoline from spraying as the cap is untwisted. Gas caps for vehicles operating on E85 fuel use the Flex gas cap, which is specially made with a bright yellow covering to remind consumers of the special fuel required. Gas caps can also be locking or non-locking.
- Owners of late-model vehicles should be aware that failing to screw the gas cap until it clicks at least three times can cause the car to display a "Check Engine" warning. Some car models will continue to display the warning until it is reset by the dealer.
