Aerodynamic drag tests were performed on a tractor-trailer combination and a box-shaped ground vehicle using the coast-down method on a smooth, nearly level runway. The tractor-trailer tests included an investigation of drag reduction add-on devices that are commercially available or under development. The box-shaped vehicle was modified by rounding the corners and sealing the undercarriage. The tests ranged in velocity from approximately 35 miles per hour to 65 miles per hour for the tractor-trailer combination and included fuel consumption measurements and one set of measurements of drive shaft torque. This paper presents the results for both vehicles, showing the effects of the various modifications on the aerodynamic drag. The effects of variation in the aerodynamic drag of the tractor-trailer combination on fuel consumption are also presented.