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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.