A large contribution to the aerodynamic drag of a vehicle (30%(1) or more depending on vehicle shape) arises from the low base pressure in the wake region, especially on square-back configurations. A degree of base pressure recovery can be achieved through careful shape optimization, but the flow structures and mechanisms within the wake that cause these base pressure changes are not well understood. A more complete understanding of these mechanisms may provide opportunities for further drag reductions from both passive shape changes and in the future through the use of active flow control technologies.In this work surprisingly large changes in drag and lift coefficients of a square-back style vehicle have been measured as a result of physically small passive modifications. Tests were performed at quarter scale using a simplified vehicle model (Windsor Model) and at full scale using an MPV. The full scale vehicle was tested with and without a flat floor. During both tests the vehicle base region was fitted with a series of low profile, horizontal slats to disrupt any steady wake structures acting close to the vehicle base surface. Force balance, static pressure and PIV measurements have been used to investigate the flow structures in the vehicle wake.This paper summarizes the results and relates the global drag and lift changes that arise from the horizontal slats to the base pressure from both model and full scale and to the PIV measurements from the model tests.