In all kinds of shapes and form road vehicles use up energy in the form of fuel. It is interesting to note that on the highway more than half of the useful energy output of the engine is used to accelerate and heat the surrounding air molecules. Aerodynamic drag is the aerodynamic force that opposes motion and accounts for a large fraction of transportation energy consumption. The main objective of this paper is to provide a solution to reduce the drag of an idealized road vehicle. This is achieved by modifying the design of vehicle by changing one parameter and then verifying mathematically whether the modification has any effect in reducing drag. Idealized vehicle geometry has been used. It has been known that the rear top slant angle of about 6 degs reduces the drag. In this research, a numerical fluid mechanics study has been carried out by keeping the rear slant angle constant, but changing the rear slant length from 50% to 10% of the rear half of the top surface. The computational part consists of numerical simulation of the flow around the body employing CFD (Computational Fluid Dynamics) techniques. Computations were carried out with and without ground effects. It was observed that drag coefficient reduced as the slant length is reduced from 50% to about 80% and then the drag coefficient starts increasing with rear slant length from about 80% to 90%.