[neon_rt]

LOL.
In the Aero tests that I've seen, removing the tailgate very seldom resulted in better mileage. Replacing the tailgate with a net results in the worst Aero drag.
Placing a toneau cover on the bed gives 50/50 results (some better, some worse). Having the tailgate in place causes a high pressure area in the pickup bed. That high pressure area keeps a vacuum from forming behind the cab.
In most cases the air hitting the tailgate results in less Aero drag than removing the gate and causing an Aero drag condition behind the cab.
The best results I've seen is to "shorten" the bed by placing a hard cover 1/4 to 1/3 the length of the bed at the tailgate. You can experiment with the length of the cover and either walling in the front side or leaving it open.
The plans I saw called for walling in the front side of the cover to make a nice 2-3ft covered storage compartment between the wall and the tailgate.
You can easily make a plain plywood device and do your experiments. After you get the measurements right you can make a permanent solution from better finish materials. It helps if you have an instant MPG readout, you can find one at scanguage.com .