A testing and analysis method has been developed for the evaluation of the aerodynamic and rolling resistance coefficients of a vehicle, under windy weather conditions and on a road with some variation in grade. Over 100 coastdown runs have been collected and analyzed for the Transport Canada Nexus research prototype. Aerodynamic and rolling resistance functions have been found for the test vehicle for conditions from zero to over 30 degrees yaw, noting a characteristic in which aerodynamic drag decreases with yaw angle for Nexus. In addition, the effects of vehicle centripetal force due to road curvature in the vertical plane, lift force, velocity dependencies of rolling circumference and rolling resistance are discussed, and all are found to be negligible. The data analysis method developed, uses a computer-based simulation/optimization technique, which is described. This new resistance analysis technique will enable vehicles to be studied under typical non-ideal road and weather conditions.