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The subject is treated on the basis of detailed engineering test data regarding components and parameters of 1975 autos. The applicability of the simulation approach is examined by extensive comparisons with integrated vehicle test results. It is found that fuel economy, for the EPA driving schedules, is adequately simulated with 5% to 10% uncertainties. Uncertainties of the same magnitude are also encountered in performance simulations. Larger uncertainties are evident in the simulation of emissions. NOx prediction has an uncertainty up to 25% but no significant bias, while CO and HC are very substantially over-predicted and under-predicted respectively. Excepting HC and CO, several applications are made in the evaluation of sensitivities to various auto components and parameters. Evaluations are made of changes in auto weight, engine displacement and rear axle ratio, considered individually and in combinations.