Induced Resistance
The induced resistance is a part of the vehicle's pressure resistance. Air pressure differences between the vehicle's upper and lower side produce cross-flows which form two large longitudinal swirls along with the roof flow.
Usually the velocity of air-flow above the vehicle is larger than under the vehicle (opposite in the Formula-1 car), leading to the formation of swirls by induced flow.
In their immediate vicinity such swirls induce low pressures. The "dead water area" at the rear end is extended and thus leads to an increased pressure resistance.