Tailgate up or down? The television show Mythbusters is sometimes interesting, sometimes silly but I caught one yesterday addressing this "what everyone knows" advice.
The often repeated premise is that an open pickup truck bed is bad for gas mileage because it catches the air and creates drag, so the easy, zero cost method for increasing mileage by reducing drag is to drive with the tailgate down thereby letting the air escape. Seems logical, right?
The two hosts drove two Ford F150s, as identical as possible, filled up with identical amounts of gas, one tailgate up, one down, until they ran out of gas. Driving one behind the other, you guessed it, tailgate down went dry first! The tailgate up truck went 30 miles further. Hmm ...
Searching for an explanation they built a fluid flow simulation of a wind tunnel with a model truck. The answer lies in the fact that an open bed with a closed tailgate creates a rotating bubble of air in the bed that remains trapped there. Air coming over the top of the cab is kept out of the bed by this bubble and drops down behind the truck. Open the tailgate, the bubble disappears and air over the cab drops down faster, hitting the bed and creating drag. This explanation agrees with designers from the auto manufacturers who say the same thing. Who knew? They didn't address the bed cover or no bed cover question which I presume will be coming up next season.