Large commercial vehicles (e.g. truck-trailer combination), due to their function, are bluff bodies by nature. The truck-trailer combination usually gives a drag coefficient that is less than that for a trailer alone. The interference between the commercial vehicles and other small vehicles is of primary importance especially when moving in driving tunnels that are characterized by their limited space. 1/19-scale models for commercial vehicles are employed in the experimental work performed here. Three-dimensional pressure measurements are recorded for analyzing the aerodynamic behavior and calculating the drag coefficient. Add-on devices (deflectors) are used. Measurements are supported by both numerical calculations and flow visualization. Results show that add-on devices play a very important role in improving the aerodynamic characteristics of commercial vehicles.