GM proves that tailgate up is more efficient
Gm tested its 2014 GMC Sierra more extensively in its wind-tunnels than pickup truck before it. I can only assume that this extensive testing also extends to the 2014 Silverado. What they found is that the truck is actually more efficient with its tailgate up. They found the air traveling over the cab falls downward and then pushes forward against the truck. This effect will not occur when the tail-gate is lowered.
GM engineers also cautioned against aftermarket nets or “aerodynamic” aftermarket tailgates. They simply aren’t as efficient as leaving your stock tailgate up. They also offered up some interesting advice as far as upgrades that do help. A soft Tonneau cover helps to smooth the airflow over the truck, more so than a hard one because “they form to how the air wants to flow”. Another worthwhile upgrade GM’s engineer said is tube-style running boards to improve the drag coefficient of the truck. He mentioned that fully integrated, flush-mount running boards are best.