Here are some things I learned while working the aeros on my 928.  We had a
windtunnel readout from the Porsche factory that showed the car, in
silhouette giving all the high pressure (read: drag) and the low pressure
(again: read drag through turbulence)  When looking for speed, which
translates to miles per gallon in a way, anything that disturbs the flow at
the front end of the object will  ruin it's smooth passage all the way to
behind the object..  My mentor, who is an engineer and a member of the
exclusive 200mph club in his 88 Porsche 928 STREET car..He would, when
running in those open road races at ~200mph, use three rolls of racer's tape
on the front of his 928...smoothing the seams in the hood, taping flat the
parking lights, covering over the weird fold down headlights, etc.  I used
to laugh, but as I began running over 150 myself, it became evident he knew
what he was doing...
  On one top speed run near Battle Mt. Nv. I forgot to take off my tiny
little roof top radio ant..About 4 inches long right above the windshield at
a raked back angle as stock..That was good for 11mph of drag, since it
created a vortex (for want of a better word) that disrupted the air over the
rear deck spoiler, etc...
  So you want to get the front of the van smooth...But yes, that big square
back end sucks, big time...
.Hey, idea!  I could duplicate the airflow I had in  my South African
splittie in it's old hippie rockclimber days. As we went through the central
Cali. valley in July we ran the front windshields wide open (they hinged at
the top with adjustable slides and knobs) and the rear hatch..wide open...We
wore our Smith goggles to keep the bugs out...That might reduce the frontal
area and decrease the drag off the back...In that trim, I could get that
36hp screamer up to 59mph..on the flat..
  Don Hanson