Ethanol
Alcohol-based ethanol is made from distilled starch crops such as sugar cane, corn, barley and wheat. Combined with gasoline it makes E85, which can fuel cars known as flexible fuel vehicles (FFVs). Ethanol's benefits are that its source can be grown, making it renewable and vehicles fueled by ethanol usually produce lower carbon monoxide and carbon dioxide emissions. Some environmentalists point out, however, that some companies producing ethanol are releasing volatile organic compounds and carbon monoxide into the air during production.

Natural gas
Natural gas is made up mostly of methane with other hydrocarbons mixed in. Natural gas is attractive as an alternative fuel source because it burns cleanly and can be produced domestically. The fuel can be stored as compressed gas or in liquid form and can also be mixed with hydrogen to create a fuel blend.

Hydrogen
Hydrogen is the lightest alternative fuel source, allowing for greater ease in transportation and storage. The gas is being used experimentally for fuel cell vehicles and combustion engines. Benefits of hydrogen fuel are that it is an easily renewable fuel source that can be produced in a variety of methods and its use may improve air quality and reduce greenhouse gas emissions. Hydrogen fuel plays a big part in President Bush's plan to reduce dependence on imported oil.

Biodiesel
Biodiesel can be created from vegetable oils, animal fats, or recycled restaurant greases and can be combined with petroleum diesels to create fuel that emits fewer pollutants. Pure biodiesel or blends with a high concentration of biodiesel can be used in some engines made after 1994 with certain modifications required in some cases, but engine manufacturers are concerned about the fuel's long-term effect on the engine and whether it is suitable for wintertime use.

Electricity
Electricity stored in batteries is the only method for vehicles that provides direct mechanical power rather than providing power through chemical energy and combustion. Since the process takes place without combustion, there is no pollution emitted. Batteries for electric vehicles have a limited storage capacity, but they can be recharged from the existing power grid or through solar or wind energy.

P-series fuels
P-series fuel is the only new fuel source to be recognized under the U.S. Energy Policy Act since the Act was passed in 1992. The blend of natural gas liquids, ethanol, and the biomass-derived co-solvent methyltetrahydrofuran is not currently widely used or mass produced.