Exhaust emissions from in-use diesel trucks and buses can be reduced by application of retrofits consisting of new parts and adjustments. The results of fleet test demonstrations of two retrofit kits, one for 2-stroke diesel-powered buses and the other for 4-stroke diesel trucks, are described. Except for the catalytic muffler, the components and adjustments composing the General Motors environmental improvement proposal kit for General Motors city buses were found helpful in reducing exhaust odor, smoke, and certain gaseous emissions. The turbocharger kit and adjustments marketed by Cummins Engine for its NHC 250, an in-use naturally aspirated truck engine, was likewise found to reduce visible smoke satisfactorily. The fleet test data are from three city buses operated for two years and three intercity truck tractors operated for eight months.