Aerodynamic vehicle design improvements require flow simulation driven iterative shape changes. The 3-D flow field simulations (CFD analysis) are not explicitly descriptive in providing the direction for aerodynamic shape changes (reducing drag force or increasing the down-force). In recent times, aerodynamic shape optimization using the adjoint method has been gaining more attention in the automotive industry. The traditional DOE (Design of Experiment) optimization method based on the shape parameters requires a large number of CFD flow simulations for obtaining design sensitivities of these shape parameters. The large number of CFD flow simulations can be significantly reduced if the adjoint method is applied. The main purpose of the present study is to demonstrate and validate the adjoint method for vehicle aerodynamic shape improvements. Although steady-state Reynolds Averaged Navier Stokes (RANS) was used as a ‘primal’ solution for adjoint-based shape changes, a fully transient Detached Eddy Simulation (DES) was applied as the baseline and final flow solutions for improved flow accuracy. This type of analysis offers a more accurate flow modelling option, especially in cases where boundary layer separation occurs. The DES validation run was performed in the final stage of this study in order to confirm the drag coefficient reduction of ΔCD = -0.005. The hardware validation results with a reduced scale clay model in a wind tunnel will be reported in future publications.