In this paper we present the work which was done at Shanghai-VW for using computational aero-acoustic (CAA) simulation in the vehicle development process to assess and improve the buffeting behavior of a vehicle when the rear side window is open. In the first step, a methodology was established and validated against wind tunnel tests using a Sedan. The methodology consists of a calibration of the CAA model to represent the properties of the cabin interior of the real car in terms of damping, wall compliance and leakage followed by CAA simulations of the full vehicle at different wind speeds to obtain the transient flow field around the exterior shape and inside the passenger compartment. The interior noise spectra are directly calculated from the transient pressure inside the cabin. For validation, frequency and level of the main buffeting peak at different wind speeds were compared to respective measurements taken in the aero-acoustic wind tunnel of the Shanghai Automotive Wind Tunnel Center. After results with good accuracy were obtained, the simulation methodology was applied to evaluate the effect of various design changes on the buffeting behavior. The successful execution of the proof of concept type project showed that CAA simulation is a viable tool for assessing and improving the buffeting behavior of vehicles in the vehicle development process.