The optimization of aerodynamic drag represents an important research area for the fuel consumption reduction of heavy duty commercial vehicles. Today's design of tractor-trailers is significantly influenced by legal conditions regarding the vehicle dimensions and the provision of a maximum transportation volume. These boundary conditions lead to brick-shaped trailer outer geometries, especially at the rear ends. That is the reason why the investigations of aerodynamic optimization of commercial vehicle trailers are predominantly restricted to detail measures up to now. The present publication treats the aerodynamic characteristics of general modifications on the outer contour of long-distance haulage trailers in regard of reducing the drag resistance and, thus, potentially also the fuel consumption in highway traffic. A new approach for the realization of a variable outer contour of trailers provides the possibility to adjust the rear end to an aerodynamically optimized shape. The variable adjustment of the sidewalls and the top of the trailer's rear end can take the actual space requirements of the transportation load into account; by this an optimal transportation efficiency is guaranteed. The research work is based on a generic, virtual 3D-CAD semi-trailer truck, which displays the characteristics of commercial vehicles and serves as a basis for several modifications. All geometrical modifications at the virtual reference vehicle are investigated and assessed by 3D-computational fluid dynamics simulations. The achieved reduction of drag leads to a decrease of fuel consumption, which encloses both, optimized operating costs for carriers as well as reduced CO2 emissions which support the intentions of environmentally friendly road transport.