A survey of the CNG compositions within NGV driving range of Houston was performed. It was found that the statistics for the Texas CNGs were very similar to those from a previous national survey Based upon the present survey results, two extremes of CNG composition were chosen for a study of the effects of composition on emissions, fuel economy, and driveability. Two other CNG compositions were also included to provide for comparisons with the recently completed Auto/Oil Air Quality Improvement Research Program (AQIRP) and to extend the AQIRP database. One of the vehicles used in the AQIRP study was also used in the present investigation. Correlations were investigated for the relationships between the CNG composition and tailpipe emissions, fuel economy, and driveability. From the perspectives of emissions and fuel economy, the most important fuel properties are the mass-based Heating Value, the energy density per cycle, the Wobbe Number, the fuel methane-to-hydrocarbon percentage, and the product of the specific gravity and the mass-based Heating Value. Relatively small changes in the fuel CH4/HC percentage can have an important influence on the tailpipe NMHC emissions, the NMHC/THC ratio, and the HCHO/NMHC ratio. It is also concluded that substitution of CNG for gasoline should produce a greenhouse benefit of ∼19%. Finally, the effects of CNG composition on driveability are minor.