Both field research and certification data show that the lean burn natural gas powered spark ignition engines offer particulate matter (PM) reduction with respect to equivalent diesel power plants. Concerns over PM inventory make these engines attractive despite the loss of fuel economy associated with throttled operation. Early versions of the Cummins L-10 natural gas engines employed a mixer to establish air/fuel ratio. Emissions measurements by the West Virginia University Transportable Heavy Duty Emissions Testing Laboratories on Cummins L-10 powered transit buses revealed the potential to offer low emissions of PM and oxides of nitrogen, (NOx) but variations in the mixture could cause emissions of NOx, carbon monoxide and hydrocarbons to rise. This was readily corrected through mixer repair or readjustment. Newer versions of the L-10 engine employ a more sophisticated fueling scheme with feedback control from a wide range oxygen sensor. Comparative testing using the 5-mile route was performed in Sacramento, California on Class 8 tractors powered by these L-10 engines, operating on liquefied natural gas (LNG) and similar tractors with M-11 diesel engines. Also, using the CBD cycle, a fleet of Cummins L-10 compressed natural gas (CNG) powered buses in Garden City (Long Island), New York, was compared to a diesel-powered fleet in Cincinnati, Ohio. Data confirms the average low production of NOx and PM, but three higher NOx emitters were found in the Long Island fleet. Energy-equivalent fuel consumption for the Sacramento trucks was 29 % higher for LNG than diesel, based on carbon content of the vehicle emissions, while for the new buses the CNG fuel consumption was higher by 28 %.