Propane

Propane is one of the leading alternative fuels in the United States. Known as liquefied propane gas (LPG), propane is the third most common vehicle fuel in the nation behind gasoline and diesel. Propane fuels 270,000 vehicles in the U.S. and roughly 10 million worldwide. A study by the Environmental Protection Agency shows propane vehicles produce up to 90% less carbon monoxide than gasoline engines and 50% fewer toxins and smog-producing emissions. Propane is limited to commercial vehicles, but vehicle conversions (upwards of $4,000) are available. Propane has 25% less energy content than gasoline.

Pros and Cons: Cleaner for the environment; less carbon deposits result in less engine wear. But filling stations are rare, and decreased energy content hurts affordability.

Hybrids

Hybrids aren't exactly "alternative fuel" vehicles, but they do conserve a significant amount of gasoline. A hybrid engine switches back and forth between gasoline and electric power depending on need. Fuel economy is increased with use of electric power, making hybrids more fuel efficient than traditional gas vehicles. Batteries recharge when the fuel engine is in use, so electric plug-ins aren't necessary.

Pros and Cons: Hybrids are widely available in dozens of models and improve gas mileage, but cost significantly more than similar gas-only models and still rely on gasoline as a primary fuel.

Electric

As with other electric devices, electric vehicles are powered by on-board batteries. Plug in the vehicle overnight to charge the battery. No gasoline is needed for all-electric vehicles. The Chevy Volt, due out sometime in 2009, will have a gasoline engine that takes over when the battery life is exhausted.

Pros and Cons: Extremely clean, economical form of transportation, but its current maximum range of 40-50 miles hurts practicality.