Trucks in the medium/heavy truck category include tractor trailers, also known as combination trucks, and single-unit trucks. In the most recent year for which data are available, combination trucks made up only 1% of the vehicle population but were responsible for 6% of all vehicle miles traveled (VMT) and 17% of vehicle fuel use. Single-unit trucks were 3% of the vehicle population and 3% of VMT, while making up 9% of vehicle fuel use.
Source: U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 2017, Washington, DC, March 2019, Table VM-1.