[COcyclist]

The proper angle has to do with the relationship between the roof of the car and the Kammback and that doesn't change whether loaded or not. The fact that it is bowing up in the middle suggests that you have created an airplane wing and you DO have attached flow. As others have suggested you are creating lift, but also reducing the size of your trailing wake. Are you driving to set a land speed record or driving for economy? Driven conservatively, gravity will keep your tires in contact with the road