The Air Tab idea is sound. Will it produce a noticable increase in fuel economy? I don't know.
Basicly, the air flowing over a truck gets ripped into the low pressure zone behind the trailer, and between the truck and trailer. The truck has to tow all that swirling turbulance down the road with it.
The air tap is just a jump ramp for air, to move some of that air out so that the turbulant zone is a little smaller. Pretty basic aero stuff actually. Shaping the truck so that air under the truck could flow better up between the truck and trailer to fill that area would also help. (smoother parts etc.) Maybe better, longer, softer, rubber skirts on the back of the cab as well.
Having a low pressure zone following a vehicle, creates a suction that drags it back.
This is why large artillery rounds have a base bleed burning fuel at the square base of the round as it flys. Not to propel it, but to fill the void of air behind it with the burning air from the base bleed, filling the low pressure zone. A tear drop shaped round would be ideal, but you need a flat surface on the shell for the explosive to push against.
http://en.wikipedia.org/wiki/Base_bleed
Also note that the trailer skirts are narrower at the front than rear. The rear should have a lower pressure than the front. If done right, it could allmost be like a giant diffuser, increasing the trailers traction by reducing lift.
I'd like to try to run an empty pipe from the high pressure zone over the cab, in front of the top fairing, though the interior to the rear of the cab. Essentially a pipe that allows air to flow from the trucks front and to fill the low pressure area behind the cab. Having a big ole pipe splitting the cab kinda sucks, but if it's high enough maybe it could be viable, or at least a neat experiment. esentially the same idea as putting your AC intake at the base of the windshield. Because there's good air pressure there.
