The Ahmed body, a widely used simplified vehicle model, forms the basis of this study, which systematically compares the aerodynamic performance of vertically mounted rigid and flexible rear flaps using computational fluid dynamics. The analysis examines drag reduction capability, flap deflection, wake suppression, and stress distribution on flexible flaps. Flap lengths of 0.1H, 0.2H, and 0.46H (H: frontal height of the Ahmed body) are investigated. For rigid flaps, the 0.46H configuration matches the experimental result. The flow control reduced drag by 3.8%, while the 0.1H flap, unique to this study, achieved a greater reduction of 9.9%. The 0.2H flap showed no benefit. In contrast, flexible flaps did not yield drag reduction, highlighting the significance of adding reinforcing cylinders to improve stiffness and performance.