Aerodynamic drag is the most dominant resistive force to the motion of a vehicle, several methods of drag reduction have been applied over the years; these mainly include shape modification and add-on devices. A novel method to improve drag response of a truck is to modify the flow so that the flow remains uniformly attached to the vehicle. This will result in a smaller wake of the vehicle and consequently reduces drag. The present study investigates the feasibility of employing one such flow modifying technique. Surface roughness has been systematically introduced on the tractor-trailer unit in the form of spherical-shaped structures and the drag of the vehicle has been measured. Computational Fluid Dynamics (CFD) based techniques have been employed to numerically simulate the flow of air over the tractor-trailer unit. The results show that the surface roughness parameters affect the overall fluid flow around the truck-trailer unit, and hence have a significant effect on the aerodynamic drag force of the vehicle. Drag force has been observed to increase by 1% when valleys were introduced as surface roughness. However, drag reduction of 1.9% has been recorded in case of roughness peaks. The results suggest that although the peaks act as a restriction to the local flow, they contribute more towards reducing the overall drag. In case of the roughness valleys, the drag force increases because these valleys introduce more non-uniformity in the flow without any benefit in drag reduction.