The current trend towards energy efficient commercial vehicles requires a substantial improvement in their aerodynamic performance.This paper describes the design methodology for a new roof fairing design with integrated ducts and the predicted effects of the final design on downstream flow. It also provides a baseline comparison with the fairing of a commercial platform and highlights the advantages of using rapid prototyping technologies to test aerodynamic improvements on commercial vehicles.By integrating into the design of a thermoplastic roof fairing ducts that divert and speed-up air flow it is possible to obtain reduction of drag in the trailer gap and alter the trailer wake favorably. The resulting decrease in yaw-averaged overall drag coefficient is of 5.8%. This translates into an improvement in fuel efficiency of 2.9% when compared to the baseline.The design optimization was performed using parametric variation of a computational fluid dynamic model at zero and six degree yaw. The resulting design will be validated in a 1/8th scale wind tunnel test of a commercial vehicle while the predicted pressures were used as inputs to a virtual prototype analysis of wind and inertial loads on the full scale model. This allows the necessary modifications for commercial manufacturing to be implemented into the design prior to a highway test in which an additive manufactured prototype will be placed on a commercial vehicle.