Fuel economy for a light-duty vehicle going 80 miles per hour (mph) is about 27% lower than when traveling at 60 mph. Between 40 and 50 mph is the optimum cruising speed for the best fuel economy in cars and light trucks, according to an Oak Ridge National Laboratory study of 74 different vehicles. The best fuel economy is typically obtained when the vehicle is traveling at the lowest speed in the vehicle’s highest gear.
Note: The study of 74 light-duty 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.