Effects of reducing engine oil and rear axle lubricant viscosities on fuel economy were determined in EPA combined City and Highway (EPA 55/45) tests and in road tests, using four different sized cars. In EPA 55/45 tests, fuel economy rating improvements averaged about 1.5 percent; warmed-up and cold-start road test fuel economy improvements averaged about 4 and 8 percent, respectively. For a specific engine oil viscosity reduction, warmed-up road test fuel economy increased with decreasing car mass and power-to-mass ratio. Warmed-up constantspeed fuel economy improvements obtained by lowering only the engine oil viscosity were about the same as those estimated from reductions in engine friction power. However, measured fuel economy improvements with lowviscosity rear axle lubricants were inexplicably higher than those estimated.Although fuel economy results with lowviscosity lubricants were generally favorable, care must be taken to make sure the use of such lubricants will not reduce vehicle performance and durability under a variety of operating conditions. Accordingly, vehicle tests are underway.