Each vehicle reaches an optimal fuel economy at a different speed or range of speeds. A recent study by Oak Ridge National Laboratory illustrates that point with a wide range of data collected on 74 light vehicles. The figure below shows that from 50 to 60 miles per hour (mph) 26 of the vehicles experienced an 11-13% decrease in fuel economy, but one vehicle only experienced a 5-7% decrease, and two experienced a 17 to 19% decrease. From 60 to 70 mph, a fuel economy decrease of 13-15% was most common, and from 70 to 80 mph, a fuel economy decrease of 15-17% was most common.

Fuel Economy Penalty in Ten-Mile-Per-Hour Increments Notes:

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• There were only 68 vehicles used for the 70 to 80 mph analysis; six vehicles were tested only to 70 mph.
• The study included two-seaters, sedans, station wagons, sport utility vehicles, pickup trucks, and minivans of model years (MY) 2003 to 2012 with a wide variety of powertrains.
• Dynamometers were used simulating highway cruising speeds on flat roads with moderate temperatures.
• From 70 to 80 mph, there are several vehicles that show more than a 21% decrease in fuel economy; these values mainly come from vehicles with cylinder deactivation. Cylinder deactivation is a fuel saving technology that allows engines to deactivate cylinders under normal cruising conditions. The switch from using 4 cylinders at cruising speed to 8 cylinders at cruising speed causes a large change in fuel economy.

Supporting Information

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