Data from the Oak Ridge National Laboratory indicates that fuel efficiency for a midsize sedan decreases significantly as speed increases. At 55 mph, a conventional sedan achieves 45 mpg, which drops to 38 mpg at 65 mph and 32 mpg at 75 mph.
For drivers covering 15,000 highway miles annually, reducing speed from 75 mph to 55 mph can result in savings of $576, based on an average gasoline price of $4.25 per gallon. Diesel vehicles show a sharper decline, falling from 55 mpg to 37 mpg, leading to potential annual savings exceeding $800.
Hybrid vehicles are also impacted by aerodynamic drag. A hybrid midsize sedan's efficiency drops from 46 mpg at 55 mph to 33 mpg at 75 mph. Hybrids reach their peak efficiency of 55 mpg at 45 mph, suggesting that high-speed highway driving reduces the benefits of the hybrid powertrain.
While specific data for electric vehicles is limited, higher speeds reduce their range and increase charging frequency. However, EVs often feature lower drag coefficients by design, and electricity costs remain lower than those for gasoline or diesel fuel.
