In addition to an overview of the Transportation Energy Panel study, the authors present and focus on findings and conclusions of the supporting Heavy Duty Propulsion sub-panel. The parent panel on Transportation Energy R&D Goals was directed by the Department of Transportation with the participation of NASA and EPA. The report concludes that aircraft could emerge as the dominant user of fuel supplies in the future, that an assured supply of economical JP type fuel will be required beyond the year 2000 as a change in aircraft fuel is unlikely in the foreseeable future, and that accurate prediction of future modal mix and inter-modal shifts requires that the total transportation system be modeled. It is particularly the latter that is needed. Inputs to the model would consist of such items for major traffic corridors as current and projected modal capacity requirements, network structures, and the efficiency and effectiveness of alternate means of transportation (auto, rail, truck, air, ship). With such a model, investment and development strategies can be worked out for the future.