[Blacktree]

Not to be rude, but I think you're operating under a false premise. There is no "parachute effect" with the side windows open. Actually, quite the opposite. The air flowing across the window openings causes a pressure drop in the cabin. If you then open the cabin to the rear wake, it will suck in air (and dust, and exhaust fumes) from the rear wake. It will also increase aero drag, because the car is pulling air forward, into the cabin (which means the air is pulling backward on the car).

Edit: Rain deflectors also increase drag. They increase the frontal area of the car. Plus they disturb the airflow down the sides of the car.