cavelamb himself wrote:

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

I remember seeing a wind tunnel test on this many years ago (I think in
the late 70s when I was an aerospace engineering student) long before
Mythbusters existed. However, there is no way you can convince some
people that this is a myth. They will argue to the death that they gain
mileage with the tailgate down, however, the wind tunnel tests were very
conclusive. A rotating vortex is established in the truck once you get
above a fairly slow speed (I think 30 MPH or so) and this acts almost
like a large balloon in the bed of the truck and directs the airflow
over the tailgate. You can see this pretty easily from the smoke in the
wind tunnel and the drag change was noticeable also.

Not only do you not get better mileage with the tailgate down ... you
actually get worse mileage! Then again, most people can't even check
their MPG correctly...


Matt