nahfwt
Member
I actually had an opportunity to do experiments that answer the tailgate up or down question while I was in college. We had a wind tunnel and put a model pickup in it and measured the drag. We tried tailgate up and down as well as off and we had a tonneau cover we put on it. The result was the tonneau cover was the best followed by tailgate up, tailgate off was third and tailgate down was the worst. It turns out that the tailgate traps a pocket of high pressure air in the bed and forces the air to flow around it which acts like a tonneau cover. With the tailgate down it acts like a flat plate and generates much more drag. In another class our professor had a friend who was a GM engineer who came in to talk to us. I asked him about this and he confirmed that they did extensive testing in their full size wind tunnels and said that the trucks are designed to be driven with the tailgates up for maximum fuel economy.