For road vehicles, it is the drag that plays a crucial role, as due to the drag (which depends on different parameters such as shape, speed, air density etc), an additional force exists which works in the contrary direction of the vehicle motion.

There is one addition for race car design: Here also lift plays a crucial role. The car is designed in a way, that the aerodynamic lift pushes it downwards towards the track in order to increase grip and to allow for higher velocities, for example while turning.

Marek is right: while minimizing the aerodynamic drag is the most important factor (from the point of view of aerodynamics) when designing conventional cars (and one of the most important when designing aircraft), in speed/race cars it is very important also to design the vehicle (or parts of it like the forward and rear ailerons, etc.) such that it generates adequate lift, negative in the usual reference frames (lift pointing toward the Earth's center, or also called, "aerodynamic load").

Let me add some information on the origins of the aerodynamic drag. There are several possible origins for drag, which are classified as follows:

1) Parasitic (or profile) drag, Do: Two possible contributions/physical phenomena:

       1.1) Friction drag, Df: due to friction between the vehicle's surface (aircraft or car skin) and the air moving around it. This drag is due to the viscosity of the air.

       1.2) Pressure (or wake) drag, Dp: associated to the wake formed downwind the vehicle and, roughly speaking, associated to the frontal area of the vehicle, and whether it is blunt or streamlined. For aircraft, which are streamlined and with high aspect ratios, the wake drag is small (that is one aerodynamic design goal), while the wet surface (surface in contact with air, where friction is acting) is very large. Therefore, the friction drag is dominant in aircraft. Conversely, for cars, trucks, etc. the wake drag is dominant. In speed/race cars, both contributions to drag are approximately of the same order of magnitude.

2) Induced drag, Di: associated to the generation of lift: the imbalance of pressure between the upper and lower surface of a wing or aileron (necessary to create lift) causes the creation of wing tip vortices (the air in the vicinity of the tips tends to move from the regions of high pressure to the regions of low pressure). Energy from the burned fuel is being wasted for creating these vortices/providing a rotational velocity component to the air, which is not useful for the flight, but an undesired phenomenom associated to generation of lift.

3) Compressibility drag, Dc: drag due to formation of shock wave(s) when in transonic regime and above. The air crossing a shock wave experiences sudden decreases in speed, and sudden increases in pressure and temperature. Again, energy from the burned fuel is being wasted in this sudden, high increase in temperature, which is not useful for the flight, but an undesired phenomenom associated to the shock waves, which are the way air has to decelerate from supersonic to subsonic speed.

Thanks a lot both of you. It would be help me in my progress design and choose the most aerodynamic one.

As mentioned by Jose and Merek, coefficient of drag plays a vital role in aerodynamic design of vehicles. But when it comes to designing flight vehicles, ballistic coefficient also plays a vital role. For re-ntry vehicle, heating becomes a critical factor as Thermal Protection System (TPS) is designed based on the stagnation heat transfer. It is cited that failure of TPS resulted in columbia space shuttle disaster. Hence stating the importance of these other factors Hope this solves your problem to a certain extent.