The influences on fuel efficiency of viscosity difference and friction modification by colloid friction modifiers in motor oil were examined separately and in combination in a series of automobile track tests using EPA test procedures. The results of this work demonstrated the following: 1.In this test program, simply reducing the viscosity of multi-grade engine oils did not produce consistent, statistically significant fuel economy benefits. 2.The incorporation of a colloid friction modifier in reduced viscosity multigrade oils produced consistent and statistically significant improvements in fuel economy over those same reduced viscosity multigrade oils without colloid friction modification. 3.Whereas reduced viscosity multigrade oils did not show significant benefits over a commercial type SAE 10W-40 proposal S-1 oil, those same reduced viscosity multigrade oils containing colloid friction modifier demonstrated consistent and statistically significant improvements in fuel economy over that commercial oil. 4.Incorporation of colloid friction modifier in a single grade (SAE 30) oil produced consistent and statistically significant improvements in fuel economy over the commercial type SAE 10W-40 oil. Statistically valid improvements in fuel efficiency due to the presence of colloid friction modifier can be observed in this data in the general range of five to ten percent.