After the invention of the automobile, people soon realized the relationship between air resistance and the movement of cars. Over time, the improved performance of the cars from different directions, aerodynamically improvement also became the fundamental question. The researchers found that attention to aerodynamics not only achieve higher speeds, but also appropriate to reduce fuel consumption and vehicle stability. This article will examine the effect of surface roughness underside of the vehicle at some points. In this method procedure we study on two zones which have maximum probability of separation flow. These zones find on rear of wheels on floor of vehicle that air flow leaves the automobile that rough sheets was attached And the results of these rough sheets compare with case of drag and lift test in wind tunnel. Results show that between speeds 15m/s to 35m/s drag force decreases about 6 percent.