Material in between cab and bed
 

I just recently saw my friends EV2 truck and noticed that the gm factory must have gotten the idea to put a liner in in between the cab and bed for efficiency. The material looked rubbery but stiff enough to not flex with the wind has anyone tried this on here? What would be the best type of material and what kind of gains could be seen?

Foam pipe insulation works pretty well. So would those foam swim noodles you see kids playing with at pools. With the pipe insulation, you'll get more variety in size, I would think.

Some, if not all, pickup truck cabs exhaust stale air out of the cab back. I'd always want some good negative pressure "pulling" at that area; it would be part of my experimentation to determine this.

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.

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! :D