Truck Fuel Use  and Average Mileage Chart.

Beginning in 1998, the Federal Highway Administration (FHWA) used the Census Bureau’s 1997 Vehicle Inventory and Use Survey (VIUS) for its baseline estimate of combination trucks. Prior to 1998, the FHWA used the Census Bureau’s 1992 Transportation Inventory and Use Survey (TIUS) for its baseline estimates. Therefore, post-1997 data may not be comparable to 1997 and earlier years.

See the chart here