The reason I asked about air dams is that Porsche found when they started experimenting with spoilers at the rear it unbalanced the car without an air dam. Yours is between the wheelbase, so
Have you tried an half-tonneau?
http://ecomodder.com/forum/showthrea...tml#post338480
There's a chart with pickup truck bed mods (that I may not find before my browser crashes). It showed a cab wing and half-tonneau combination was ~equal to an aerocap.
Have you consulted with kach22i?
Photobucket ate all his posted pix —(
)— but Google cached this thumbnail:
Google Search Results - EcoModder.com
I've posted a screenshot of the U of Warwick study on perforated base plates. Or maybe it was Morelli's Lupo? I'll try to find something.