Newly formulated lubricants for high-speed diesels in vehicular service improved the fuel economy of a truck fleet using fuel efficient oils in controlled operations by 3 to 3.7% (confidence interval = ±1%), relative to three conventional commercial heavy duty oils. In this service, with fully warmed-up engines, multigrade SAE 15W-40 and single grade SAE 30 oils gave equivalent fuel economy. However, in a diesel car in cold-start commuter service, an SAE 15W-40 gave about a 3.8 ± 2.0% fuel economy benefit, relative to the SAE 30 version of the same formulation.The evaluation of fuel economy differences for crankcase oils in uncontrolled diesel truck fleets is often futile. The randomness of the operation produces imprecise results that prevent drawing conclusions with confidence. Controlled operation, with statistical adjustment of the data, is required for meaningful results. Care must also be taken to ensure consistency of engine conditions.The 3% benefit in fuel economy from a fuel efficient lubricant translates into large savings for highway diesel applications. A reasonable estimate for the U.S.A. is eleven million barrels of fuel saved per year.