We analyze alternative fuel/engine internal combustion engine and electric technology options that will be available over the next two decades for powering a large proportion of the U.S. light duty fleet (cars and light trucks). Lifetime private (vehicle, battery replacement and fuel) and social (externalities associated with pollutant and greenhouse gas emissions) costs for the vehicle options are estimated and compared to those of a baseline gasoline fueled internal combustion engine automobile. Technological advances continue to improve the efficiency and environmental performance of low sulfur reformulated gasoline fueled internal combustion engine automobiles. In our judgment, two fuels which may challenge gasoline for use in internal combustion engines are compressed natural gas and biomass ethanol. Compressed natural gas would lower our dependence on foreign energy supplies, would result in lower greenhouse gas emissions, and is cleaner burning than petroleum fuels. However, vehicle fuel storage and range, as well as infrastructure issues, are drawbacks. If global warming becomes a major concern, biomass ethanol provides a fuel with no net carbon dioxide emissions, although the fuel price would be more than twice current petroleum prices. Although advanced electric vehicle options such as battery powered, hybrid electric, and fuel cell vehicles promise environmental benefits such as lowering of vehicle (and upstream fuel cycle) emissions and improved fuel economy, in the near term these benefits do not justify their costs.