Kammback
A boattail, also known as a kammback can have the biggest influence in reducing the drag of a vehicle. A kammback is created by drawing out the rear of the vehicle at a slight angle down to the ground. This drawing out can reduce if not eliminate the trailing wake of the vehicle. While a full boat tail would reduce the vehicles drag the most, a partial boat tail is more commonly seen in racing, high efficiency vehicles, and trucking.
Read more about this topic: Automobile Drag Coefficient, Fabrication