A data reduction procedure is presented and sucessfully used to analyze coast-down data obtained in a windy environment to provide a measure of the aerodynamic drag on a full-scale tractor-trailer combination as a function of the yaw angle of the vehicle. Full-scale drag coefficients were evaluated for a vehicle operated in the baseline mode, and following the addition of four different drag reducing combinations, over a yaw angle range of from -10° to 10°. Comparison with wind tunnel measurements suggests that the wind-tunnel provides a reasonable simulation of the effects of winds on vehicle drag. The full-scale drag reductions measured in the presence of winds were generally lower than those found in wind-tunnel tests, except in situations where a vertical gap seal device was present. In this case, there was good agreement between the results of the coast-down tests and the wind-tunnel tests. The highest drag reductions that were measured in the presence of winds were obtained with a streamlined fairing/gap seal combination, which produced a 32% drag reduction at 0° yaw, and a 21% reduction in wind-averaged drag. The drag reductions obtained with the roof fairing, with and without gap seals, significantly exceeded those obtained with a commercially available wind deflector, with and without a vortex stabilizing device.