If you drive a pickup, common sense might lead you to think that opening the tailgate on the road is the more fuel efficient way to go. In fact, the idea that tailgates block airflow and increase drag became widely circulated, and some truck owners intentionally drive with the tailgates down to ease up on gas consumption.
Closing the tailgate actually improves fuel efficiency because it creates a type of airflow called a separated bubble within the bed of the truck. As wind rushes over the moving truck, that bubble of slow-moving air deflects it over the raised tailgate. By guiding surrounding air over and across the bed of the truck, that vortex effect prevents added drag.
However, driving with the tailgate open eliminates the bubble effect, pulling the air toward the truck bed and creating more drag rather than deflecting the wind. Some gas-conscious pickup drivers still swear by leaving their tailgates open, but science sides with leaving it up.
Driving With Tailgate Up Is Fuel Efficient : Discovery Channel