It would stand to reason that you might be able to get better gas mileage with the tailgate down on a pickup truck, right? I mean, with the tailgate up (closed) it stops the wind and creates more drag, doesn't it?
In short, no.
Long story short, the with the tailgate closed, the air flowing over the truck enters the bed of the truck and the flows back towards the cab, creating a little vortex/tornado of air that stays in the bed. This wave of air in the bed creates a natural cover for the truck bed, allowing other air flowing over the cab of the truck to efficiently flow past the bed, past the tailgate and beyond the truck, thus lowering the truck's drag. Driving with the tailgate down actually INCREASES the drag because the air "cover" is never created – air escapes right out with the tailgate down.
And, all things equal, a lower drag coefficient will contribute to better gas mileage (less resistance, less friction, less power need to maintain velocity, etc).