Tests were conducted with two tractor-trailers on an 8-mile track to determine effects of tire parameters on fuel economy. Four-tire samples of tires used on the trucks were laboratory tested to determine rolling resistance values. Laboratory data were corrected to fuel economy test conditions and compared with tire rolling resistance data extracted mathematically from fuel economy tests. Good agreement was obtained - a correlation factor of 0.957. The factor relating comparative change in measured laboratory tire rolling resistance to change in truck fuel economy was determined for selected tires. A method is presented for converting laboratory rolling resistance data to road conditions for use in truck performance calculations.