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Between 40 and 50 miles per hour (mph) is the optimum cruising speed for high fuel economy in cars and light trucks according to an Oak Ridge National Laboratory study of 74 different vehicles. The average fuel economy decrease from 50 to 60 mph was 12.4%; from 60 to 70 mph the average decrease was an additional 14%; and from 70 to 80 mph the average decrease was another 15.4%. The best fuel economy is typically obtained when the vehicle is traveling at the lowest speed in the vehicle’s highest gear.
Estimates for fuel economy by speed for an individual make/model of vehicle can be found on the FuelEconomy.Gov website at Driving More Efficiently.
Average Fuel Economy by Speed, Study of 74 Vehicles
Note: The study of 74 light vehicles included two-seaters, sedans, station wagons, sport utility vehicles, pickup trucks, and minivans for model years ranging from 2003 to 2012 with a wide variety of powertrains (e.g., 4-, 6- and 8-cylinder engines and transmissions with 4, 5, 6 and 7 speeds and continuously variable transmissions). Both 2- and 4-wheel drives were included.
Sources: U.S. Department of Energy and U.S. Environmental Protection Agency, Fuel Economy Guide, Driving More Efficiently.
Green Car Congress, "ORNL researchers quantify the effect of increasing highway speed on fuel economy." February 8, 2013.