I'm sure it depends on truck specifics. I doubt one size fits all. My recent years' experiences with two different trucks and four different caps total was that air tended to enter not exit through rear glass. Of course both trucks were extended cab models with tip out rear glass on the sides. Hence, Venturi effect when used in concert. But even when just cracking rear window alone I seem to recall net inflow. Of course that makes me wonder where cab outflow happened for balance. Dang - now I gotta experiment too!
