Today there are a large variety of drag-reducing devices for heavy trucks that are commonly used, for example, roof deflectors, cab side extenders and chassis fairings. These devices are often proven to be efficient, reducing the total aerodynamic resistance for the vehicle. However, the drag-reducing devices are usually identical for a specific pulling vehicle, independent of the layout of the vehicle combination.In this study, three vehicle combinations were analyzed. The total length of the vehicles varied between 10.10 m and 25.25 m. The combinations consisted of a rigid truck in combination with one or two cargo units. The size of the gap between the cargo units differed between the vehicle combinations. There were also three configurations of each vehicle combination with different combinations of roof deflector and cab side extenders, yielding a total number of nine configurations.The aim of this investigation was to determine the aerodynamic effects of the roof deflector and cab side extenders as a function of the type of vehicle combination. Important factors were the total length of the vehicle combination and the influence of the drag reducing devices further downstream. The investigation was performed using Computational Fluid Dynamics (CFD).The results from the investigation showed that the effect of the two drag-reducing devices analyzed was different depending on the type of vehicle combination. It was established that the roof deflector and cab side extenders were always efficient in reducing drag both in 0° yaw and 5° yaw but the magnitudes differed between the configurations. The largest effects of the drag-reducing devices were seen for the truck including the 1st cargo unit; the influence of the drag-reducing devices on the 2nd cargo unit was smaller. The aerodynamic reductions were diminishing downstream, why it was concluded that it is of great importance to improve the aerodynamic design of the rest of the vehicle to maintain the positive effects of the drag reducing devices.