An empirical model that characterizes the relationship between equilibrium vehicle emission distributions and malfunction, repair, and replacement rates by splitting vehicles into two emission categories has been developed. Gross emitters and clean vehicles are defined by the magnitude of their emissions compared to an arbitrarily chosen cut point. The model is used to predict emission changes resulting from changes in any of the three rates, by determining the new equilibrium distribution of vehicles. The emission cut point for repair is shown to establish the cost per ton of reduction, and the maximum emission reduction limit (i.e., all vehicles in the clean category), while the frequency of repair determines the amount of emission reduction up to the cut-point determined limit. (Copyright (c) 1993 American Chemical Society.)