What about making the back end into a large Kamm Tail?
http://homepage.mac.com/christopher.z/hobby/80-AMXitems/Information/production/KammbackStory.html

-------------- Original message --------------
From: John Bange <jbange@GMAIL.COM>

> On Fri, Jun 13, 2008 at 6:56 AM, Chris S wrote:
> > So you're saying reducing under-body air flow will not help a wheezing 82 hp
> > engine pushing a barn door through the atmosphere at 65mph?
>
> I reckon it'd help, but as Jim Akiba points out, the compression of
> the air flow over the top, down the sides, and under the bottom are
> rather small potatoes compared to the turbulence-induced vacuum caused
> by all that air trying desperately to re-fill 3/4 of a cubic meter of
> space across a 1.5x1.5 meter plane every 1/100th second. That
> near-flat back end is a killer.
>
> --
> John Bange
> '90 Vanagon - "Lastwagen"
> '90 Vanagon GL - "Wiesel"