JOST Dachdiffusor SDR (System Drag Reduction)
The SDR roof diffusor is an aerodynamic airflow system which reduces the decelerating air turbulences at the rear of the vehicle by more than 50%. 
JOST SDR has a positive effect in reducing air turbulence at the rear of the vehicle. The air flowing over the roof is directed through the turbulent zone by the JOST SDR diffusor, significantly reducing the drag effect caused by the turbulent air.

Besides the remarkable savings on consumption, it also reduces the respective CO₂ emissions and protects the environment. Two more benefits of JOST's SDR roof diffusor resulting from the low air turbulence behind the vehicle are most noticeable during bad weather: The rear of the vehicle remains cleaner and the view, e.g. in rainy conditions, for following vehicles is improved, as less spray mist develops behind the vehicle.

Further to reduce interference drag attempts are also being made to provide a smoother underbody structure for the car, together with suitable recessing for the exhaust system and other components wherever this is feasible.

The gap between the car underbody (undertray, front flap or forewing) and the ground
The idea behind an undertray is to use the close proximity of the vehicle to the ground, termed ground effect, to cause a venturi-like effect under the vehicle.