A computer program is used to predict the effect of engine emissions, vehicle gearing, test weight and driving technique on light duty diesel truck emissions levels. The predictions are correlated with actual tests and the vulnerability of using steady state instead of instantaneous emission data studied. An outline of the development of an engine to meet the latest E.P.A. emission level requirement is given. It is concluded that the degree of emissions control required of an engine in order to meet specified legislation depends significantly on whether that engine is to be installed in a truck or a car.