Two forms of natural gas are currently used in vehicles: compressed natural gas (CNG) and liquefied natural gas (LNG). Both are domestically produced, relatively low priced and commercially available. Considered alternative fuels under the Energy Policy Act of 1992, CNG and LNG are sold in units of gasoline or diesel gallon equivalents based on the energy content of a gallon of gasoline or diesel fuel.

Natural gas must be liquified for use in vehicles. It is an odorless mixture of hydrocarbons, mostly methane. About 30% of the energy used in the United States is natural gas, per the DOE. A very small amount of natural gas—only about two-tenths of 1%—is used for transportation fuel.