Wind tunnel experiments, with emphasis on cross-wind effects, have been used to evaluate the effectiveness of a number of commercially available devices for reducing the aerodynamic drag of a tractor-trailer combination. The evaluations included consideration of the effects of tractor type, trailer height, and the bluffness of the tractors and/or trailers. A wind-averaged drag coefficient was introduced to interpret the basic data for the prediction of average drag in a highway environment. The average drag of the base-line vehicles was found to be a strong function of the bluffness of the tractor and/or trailer, and a weak function of the tractor type. Estimates of the average fuel savings that would result from the application of the various drag reducing devices ranged from -400 to 3300 gallons per 100,000 miles, depending on the combination of tractor, trailer and device that is used.