Most computer simulation models do not predict the transient warm-up characteristics of a vehicle. This paper shows how the standard EPA fuel economy equations can be used to predict city fuel economy from only the “hot start” fuel consumption. The results of 2550 emissions certification tests on gasoline-fueled automobiles were used in the analysis. A plot of the data shows a strong linear relation between cold start fuel economy and hot start fuel economy; R2 = 99.76%. The regression equation from this comparison was then used to express EPA city fuel economy as a function of only the hot start fuel economy.