ABSTARCTStatistical analyses of extensive fleet test data from several sources with the gasoline antiknock additive, MMT, provide mathematical models for each car make that show the effect of MMT and mileage accumulation on hydrocarbon emissions. These models show: 1. Engine-out hydrocarbons increase linearly with MMT concentration in the fuel up to 50,000 miles, which is the limit of the data. 2. MMT results in enhancement of catalyst converter efficiency, which also shows a response to the MMT concentration in the fuel.