The heavy duty transport sector which uses trucks to transport cargo covers a very large business area. Due to the climate change it is crucial however, to find solutions to stay sustainable in this aggressive and fast changing market. One way to address the climate change problem is to reduce the required power for road transport. Improving aerodynamic quality, i.e. reducing drag, can contribute largely to the solution to this problem. With numerical analysis of a standard tractor-trailer combination a general overview of the flow around this type of vehicle has been obtained. The visualizations show a highly complex flow and large separated regions especially at the back and at the bottom of the trailer. The analysis indicates a large contribution of the underside of the trailer to the total drag of the vehicle. Wind tunnel experiments in the closed circuit Low Turbulence Tunnel at the TU Delft with a 1:14 scaled truck model were performed to experimentally investigate the effect of aerodynamic aids to reduce the drag of the tractor with trailer. More than 100 different aerodynamic devices were built and tested. Covering the wheels was found to be always beneficial and a full underbody is not performing as well as the straight side skirts. Modifications to these side skirts improved the drag reductions even further up to 14% which corresponds with a drag coefficient reduction of 0.062. A full-scale prototype of glass-fiber is built to execute several road tests with a tractor-trailer combination. During the operational tests, which are performed over a period of 4 weeks, an average fuel economy increase of 9% is achieved. A test day with two identical trucks (one was equipped with the aerodynamic side skirts) confirmed these results.