This paper presents the development of a computer model to simulate fuel usage and emission contributions of the past and future truck and bus population in the United States. The projected future years are beyond 1976 to 1990. The trends in vehicle population growth, yearly miles traveled and ton-miles are also calculated by the model. The model developed is flexible and brings together several technical concepts which reflect recent inputs from industry and government.The formulation of the model is based on a systems approach, in which the several submodels (the "Population," "Mileage," "Fuel Usage," and "Emission") are interrelated. The preliminary quantitative results are discussed to demonstrate the satisfactory performance of the computer model. Increased rates of dieselization are analyzed to determine their effect on reducing fuel consumption and the impact on total emission contributions.The use of the computer model to study an urban area for air quality is discussed. It is pointed out that the data availability remains a constraint for regional analysis. A case study of New York is presented to illustrate the use of the model.