The use of specially formulated engine oil is now an acceptable means of increasing the fuel efficiency of a vehicle. In principle, many of the same arguments used to justify their performance can also be applied to gear lubricants. Viscometrics and shear stability are discussed as they pertain to proper polymer selection for gear lubricant service. The fuel efficiency effects for three experimental lubricants relative to a commercial SAE 90 grade fluid were determined from a 3.1 million mile (5.0 million km) fleet test involving heavy duty trucks in normal service. The statistical techniques used to design this fleet test and analyze the data are presented, and the physical interpretation of the results is discussed. The results show that use of properly selected multi-grade gear lubricants can result in significant reduction in fuel consumption.