I would think that the cab "exhaust" would be sucked out from the wind going from below better than the wind passing by on the sides. Of course, if you have a belly pan it would only be able to pull from the top. Also, with the air movement in the bed of the truck, i'm thinking it would either suck up or push down through that slot MORE than side pressure effecting it. So, if no belly pan or aero-cap, it SHOULD just be channeled more efficiently.
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