An experimental program was conducted at the U.S. Department of Energy’s Bartlesville (Okla.) Energy Technology Center to evaluate the effects of synthetic and special additive engine and differential gear lubricants on automotive fuel economy over the temperature range encountered in the United States.Using a climate-controlled chassis dynamometer facility on two 1978 vehicles, four engine lubricants were evaluated in the 1978 Federal test procedure and in steady-state operation from cold start at 20°, 70°, and 100° F ambients. In addition, three differential gear lubricants were evaluated in steady-state operation from cold start at 20°, 70°, and 100° F ambients using one 1978 vehicle equipped with torque and revolutions per minute measuring devices, which enabled the calculation of horsepower losses in the differential and the efficiency of the rear axle in addition to fuel economy.The results from the Federal test procedure and steady-state operation showed improvements in fuel economy with the synthetic and graphite-containing lubricants on the order of 1 to 6% when compared to a common base 10W40 lubricant. The order of magnitude varied with the ambient temperature and test vehicle. In steady-state operation, the fuel economy associated with the synthetic and friction-modified 75W gear oils increased at all ambients only during vehicle warm-up from cold start. Under fully warmed-up conditions there was little or no change in fuel economy.