Two modern light-duty passenger vehicles were selected for chassis dynamometer testing to evaluate differences in performance end efficiency resulting from CNG and gasoline combustion in a vehicle-based context. The vehicles were chosen to be as similar as possible apart from fuel type, sharing similar test weights and identical driveline configurations.Both vehicles were tested over several chassis dynamometer driving cycles, where it was found that the CNG vehicle exhibited 3-9% lower fuel economy than the gasoline-fueled subject. Performance tests were also conducted, where the CNG vehicle's lower tractive effort capability and longer acceleration times were consistent with the lower rated torque and power of its engine as compared to the gasoline model.The vehicles were also tested using quasi-steady-state chassis dynamometer techniques, wherein a series of engine operating points were studied. When the indicated thermal efficiency at each point was calculated, it was found that the CNG vehicle typically exhibited lower thermal efficiency.Several operating points were chosen for further characterization of engine efficiency and combustion behavior, including an analysis of losses. Though the CNG engine had better theoretical efficiency potential, the losses suffered by this engine were repeatedly more significant than those suffered by the gasoline engine. As a result, the CNG engine was typically less efficient. There were a number of factors contributing to this phenomenon, including compression ratio, fuel properties, ignition and combustion timing and phasing, as well as EGR rates.