This paper presents the results of Argonne National Laboratory's assessment of the fuel-cycle energy and emissions impacts of 13 combinations of fuels and propulsion systems that are potential candidates for light-duty vehicles with tripled fuel economy (3X vehicles). These vehicles are being developed by the Partnership for a New Generation of Vehicles (PNGV). Eleven fuels were considered: reformulated gasoline (RFG), reformulated diesel (RFD), methanol, ethanol, dimethyl ether, liquefied petroleum gas (LPG), compressed natural gas (CNG), liquefied natural gas (LNG), biodiesel, Fischer-Tropsch diesel and hydrogen. RFG, methanol, ethanol, LPG, CNG and LNG were assumed to be burned in spark-ignition, direct-injection (SIDI) engines. RFD, Fischer-Tropsch diesel, biodiesel and dimethyl ether were assumed to be burned in compression-ignition, direct-injection (CIDI) engines. Hydrogen, RFG and methanol were assumed to be used in fuel-cell vehicles.Impacts were analyzed under alternative scenarios of potential 3X vehicle market penetration. Profiles of 3X and conventional vehicle stocks were then used to estimate fuel supply requirements and emissions produced by all light-duty vehicles (both 3X and conventional) expected to be on the road in each year of the analysis. Energy consumption, and emissions of criteria pollutants and greenhouse gases were estimated for upstream fuel processing/production as well as for vehicle operation. Emissions of criteria pollutants were further disaggregated into urban and nonurban components.Results show that the fuel efficiency gain by 3X vehicles translates directly into reductions in total energy demand, fossil energy demand, and greenhouse gas (primarily CO2) emissions. The combination of fuel substitution and fuel efficiency results in substantial petroleum displacement and large reductions in urban emissions of volatile organic compounds and sulfur oxide for all propulsion system/fuel alternatives considered. Although urban emissions of particulate matter smaller than 10 μ m increase for CIDI engines operating on RFD, biodiesel, and Fischer-Tropsch diesel, such increases do not occur for CIDI engines operating on dimethyl ether. Fuel-cell vehicles produce large reductions in urban emissions of nitrogen oxide and carbon monoxide; compression-ignition engines operating on RFD, dimethyl ether, Fischer-Tropsch diesel or biodiesel also yield ubstantial reductions in urban emissions of carbon monoxide.