Appropriate chassis dynamometer simulation of road power for truck tractor-trailers and buses were required for emissions evaluations. To establish such simulations, the power required to operate vehicles over a roadway (speed-power relationship) was determined for two truck tractor-trailers and one city bus. Results of these determinations, along with data reported in the literature, were used to determine the power to be absorbed by a chassis dynamometer to simulate On-road driving of trucks and buses. The chassis dynamometer is being used in the subsequent phases of this study involving emissions evaluations of heavy duty vehicles.THE PURPOSE OF THIS PAPER is to describe the findings associated with road power determination and simulation for heavy-duty trucks and buses. Included is a general discussion of road power, along with the results of evaluations on the road and on the chassis dynamometer. The results have been used to define the appropriate amount of power to be absorbed by a chassis dynamometer to simulate on-road driving of trucks and buses. Appropriate road power simulation is important for meaningful emissions evaluations on a chassis dynamometer. Subsequent studies, associated with this program, involve emissions evaluations on a chassis dynamometer.