Many a bet has been made, and many a friendly, and not-so-friendly, argument has been waged over whether you get better gas mileage in a pickup truck with the tailgate up or down. Even those gurus of auto advice Click and Clack were thrown by it.
The intuitive answer is that it is better to open the tailgate so the air can rush out the back, or use one of those tailgate nets which lets air flow through, supposedly cutting down drag.
Well, as aerodynamics engineers have worked to streamline vehicles, they have certainly looked into the tailgate question. When they did drag tests on pickups, what they found out surprised even them. They discovered there was less drag with the tailgate closed!
A closed tailgate sets up a bubble of air that slowly circulates in the bed of the truck. Air coming over the cab of the truck treats the bubble as a smooth cover over the bed, and passes over this "cover" without ramming into the tailgate.
Opening the tailgate breaks up this smooth bubble of air. The air then rushes over the cab, and quickly down into the bed. This creates a lot of turbulence, which results in more drag. So technically speaking, with the tailgate open the shape of the truck is not as aerodynamic as it is with the tailgate closed.
So, gas mileage is better with the tailgate closed. Actually you probably wouldn't notice much improvement at lower speeds, but at highway speeds of sixty miles-per-hour and above you should get slightly better mileage.
Also, the truck is structurally stronger with the tailgate up, so that's two good reasons to keep it closed. Well, maybe a third would be to keep the trash you're hauling from spilling onto the road, but that's another story.