Aerodynamics acording to Myth Busters!

It is more fuel efficient to drive a pick-up truck with its tailgate
down, rather than up.

Busted

Driving with the tailgate down actually increased drag on the pick-up
and caused it to consume fuel faster than the identical truck driven
with the tailgate up. It was later revealed that the closed tailgate
creates a locked vortex flow that created a smoother flow of air over
the truck. With the tailgate down, the trapped vortex was dissipated and
the drag increased.




http://en.wikipedia.org/wiki/MythBus...%29#Blown_Away