This paper presents two analytical procedures for estimating passenger car fuel economy over the Federal Urban, Highway, and Combined driving cycles as functions of basic vehicle and engine characteristics. The first procedure is based on algorithms developed by regressing fuel economy test results from 1059 Model Year 1984/1985 certification vehicles. The second procedure is based on a detailed computer simulation of vehicle and engine operation over these driving cycles. In this manner, changes in specific parameters of spark ignition and diesel-engined vehicles are related directly to changes in fuel economy. From the computed data, an equation is generated by regressing the fuel economy results.