Tests were performed with tractor-trailers on a 13-kilometer track to determine effects of tire parameters on fuel economy. Radial tires gave up to nine percent advantage over bias tires when used in all wheel positions of a fully loaded truck. Using radial rather than bias tires on the trailer only provided the largest fuel economy gain of any axle position. Radial tire tread compound affected gains over bias tires significantly. Laboratory tire rolling resistance data provided a ranking of tread compounds that agreed with the fuel economy test. Track surface roughness was a factor in evaluating test results.