A 2.3L engine dynamometer test procedure that measures the effects of engine oils on fuel efficiency has been developed that a) generally agrees with the ASTM Five-Car test, b) has good test repeatability and c) is capable of detecting small differences among test oils with high statistical confidence. Factors in a lubricant affecting fuel efficiency such as SAE viscosity grade, VI improver, detergent-inhibitor (DI) package and friction modifier selection were investigated in the 2.3L engine dynamometer test. A general trend of improved fuel efficiency was found with reduction in single-grade and multigrade oil viscosity. VI improver selection was found to have a significant effect on the fuel efficiency of multigrade oils. In some cases the difference in fuel efficiency among multigrade oils containing different VI improvers was about the same as the gain in fuel efficiency from reducing SAE grade from an SAE 10W-40 to an SAE 5W-30. Results show that by careful selection of the VI improver and DI package it is possible to formulate multigrade oils that exceed the requirements of the current ASTM Energy-Conserving Engine Oil classification.