In a study to determine the potential environmental effects of a compressed-natural-gas-fueled (CNG) passenger car, engine-dynamometer tests were conducted with a 2.8-L engine fueled with a pre-mixed charge of either methane (CH4) or actual CNG. At the base compression ratio (CR) of 8.9:1, stoichiometric versus lean emission control strategies were explored. CR was then raised to 11.5:1 and 14:1. Estimates of tailpipe exhaust emissions and fuel consumption on the US urban and highway driving cycles were made based on steady-state engine-dynamometer test results. Speciation of the exhaust emissions provided indication of the potential to meet future exhaust emission standards. Unregulated emissions including toxics and greenhouse gases were examined as well as the potential to form ground-level ozone (O3).