Eight engine oils were evaluated in four GM vehicles in standard EPA fuel economy (FE), vehicle-dynamometer tests. The results were compared with the FE obtained with a standard ASTM reference oil (BC). The viscosity and the friction modification effects of engine oil on vehicle FE were quantified.Combined FE performance in the vehicles ranged from almost 2 percent improvement for an SAE 0W-10 oil, to over 1.5 percent poorer FE than the reference oil for an SAE 10W-40 oil. FE in three engines (3.1L, 3.8L, and 2.3L) showed a strong dependence on the viscosity of the oil (HTHS at either 100° or 150°C). This dependence was stronger during the city portion of the EPA test (lower temperatures) than the highway portion (higher temperatures). For the 5.7L engine no significant effect of oil viscosity on FE was observed although the highest FE seemed to be obtained at an HTHS (at 150°C) viscosity near 3.1 cP.All four engines responded strongly to both of the friction modifiers used in this study (an organic and a Mo-type). The average (over all engines) combined FE improved by 1.6% with the Mo friction modifier and by 1.5% with the organic friction modifier. The effect of both friction modifiers was higher for the highway portion of the test than for the city driving schedule.