Although considerable efforts have been made with respect to the reduction of fuel consumption of trucks during the last decades, the diminishing natural resources as well as the evolution of the truck traffic require continuous improvements in the field of aerodynamics. Indeed, the forces generated by the air on the trucks may originate, depending on weather, road type, truck type, dimension, etc., up to 50% of the fuel consumption.In order to analyze the influence of proportion variations (mainly related to the length) and add-on devices on the aerodynamic performance of a truck, a representative model was first generated. This simplified geometry of a tractor-trailer was based on the geometrical data of six European OEMs: Daimler, Iveco, and MAN (tractors), Kögel, Krone and Schmitz Cargobull (trailers). The model included a reduced level of details (exterior mirrors, wheels, simplified underbody and engine block).The following processing chain was used for the numerical investigations. The grid generation tool, Spider, is an octree-based preprocessor producing boundary fitted, locally refined, conformal meshes, including hexahedral cell layers close to walls. The computational grid obtained was then used for the spatial discretization of the Reynolds-Averaged Navier-Stokes equations that are solved in parallel using the open-source software OpenFOAM®. For comparison purposes the generic truck was built on a scale of 1:2.5 and investigated in Daimler's wind tunnel in Stuttgart parallel to the numerical study. Both experimental and numerical results were found to match in terms of flow topology and drag coefficient.Furthermore, an optimization analysis based on the generic body was carried out, where geometric variations or fittings for the front and the rear of the tractor-trailer were considered. The wind tunnel models of the trucks were made in modules to enable the experimental testing of almost all of the optimized configurations that were simulated. The numerical results showed very good agreement with the experimental data. In particular, the drag improvements matched satisfactorily. Consequently, this study will be extended with numerical investigations focusing on the 25.25-meter-long truck combinations (European Eurocombis).