Another myth busted on mythbusters is that pickups getter better gas
mileage with the tailgate down.  Quite the opposite - they get better
mileage with it up!  It has to do with less turbulence behind the cab.
The air flows better over the relatively stable air held in place by the
tailgate.  When the tailgate is down, lots of turbulence just behind the
cab and increased drag...

Jeff Olson
85 Vanagon