I saw some pics from a wind tunnel test on Vanagons.  The stock passenger
van is .44, the US spec Westy is .50.  I am currently driving a Westy with a
modified "aero" front end for the Westy top.  It is getting better mileage,
but due to adverse traffic/winds, I could nail down good numbers.  It is
definitely getting somewhere between 5-10% better mpg.
   The Porsche 928 is a horrible example of aerodynamics, despite its sleek
look.  Its Cd was similar to a Vanagons when it first came out, but I
believe they tweeked it in the mid 80's so it was mid/low .30s.
    Westy's sold in Europe had a bunch of variants on the top design.  One
common one was the same rear as the US model, but a sleeker front.  That is
basically what I built for the front of my Westy, although the first one is
far from beautiful.

    John