Computational Fluid Dynamic (CFD) simulations of the steady state Reynolds-Averaged Navier-Stokes equations using the second order k-ω turbulence model were performed to investigate the Drag Coefficients of the Class 8 trucks under different geometric configurations. The reduction rates of aerodynamic drag using several add-on devices such as base flap, boat tail, underbody skirt and tractor-trailer gap are investigated through the CFD simulations. Moreover, simulations by using different lengths and angles of the base flap and boat tail were conducted; the comparison indicates that the base flap design is more effective for the drag reduction. The optimized range of the length and angle of the base flap design is confirmed by a set of systematic CFD simulations. Also, a set of wind tunnel experiments had been conducted to confirm the outcomes from the CFD simulations.