The objective of the work described here is to test the performance of closed-loop controlled, heavy-duty CNG engines in-use, on fuels of different methane content; and to compare their performance with similar diesel vehicles. Performance is measured in terms of pollutant emissions, fuel economy, and driveability. To achieve this objective, three buses powered by closed-loop controlled, dedicated natural gas engines were tested on the heavy-duty chassis dynamometer facility at the Colorado Institute for Fuels and High Altitude Engine Research (CIFER). Emissions of regulated pollutants (CO, NOx, PM, and THC or NMHC), as well as emissions of alde-hydes for some vehicles, are reported. Two fuels were employed: a high methane fuel (90%) and a low methane fuel (85%). It was found that the NOx, CO, and PM emissions for a given cycle and vehicle are essentially constant for different methane content fuels. Hydrocarbon emissions varied widely because of open-loop operation for a short time immediately after startup. In particular, several of the older model year engines operated open-loop for up to three minutes after starting, and during this period driveability was poor and hydrocarbon emissions were high. The non-methane hydrocarbon was found to be more than 99% ethane, ethylene, and propane. Newer CNG engines exhibited higher fuel economy and lower aldehyde emissions than older models. An additional study objective was to compare performance of similar diesel and CNG powered vehicles. To this end testing was performed on an additional 6 CNG buses and 3 CNG trucks, as well as 5 diesel buses and 1 diesel truck. It was found that CO, NOx, and PM emissions are substantially lower for CNG relative to similar diesel vehicles, particularly for newer model CNG engines. Idle emissions from diesel and CNG vehicles are also compared and an identical conclusion can be drawn.