Axle dynamometer tests were carried out to evaluate the effects of rear axle lubricant viscosity-temperature behavior and frictional characteristics on vehicle fuel economy. Using a Ford 9 inch 2.75:1.0 ratio axle, a set of input speed and load conditions was selected to permit simulation of the CVS and EPA highway driving cycles. Lubricant temperature was varied from -30°C to 100°C to simulate seasonal climatic effects. Data obtained for three lubricants differing in viscosity-temperature behavior were interpreted assuming a lubrication model including both elastohydrodynamic and mixed lubrication conditions. From these data, fuel economy projections were made using a vehicle simulation computer program.The results predict that improvements in vehicle fuel economy on the order of a few percent can be made at low temperatures by use of low viscosity synthetic lubricants, but only small effects are projected for the CVS and EPA highway cycles. Insufficient data were obtained to quantify the contribution of friction modifiers to fuel economy. However, the data indicate under what viscosity values and input conditions friction modifiers will become effective and suggest areas for future evaluation.