It would be logical to think that gas mileage would be better for the old pick up truck with the gate down because it would be a more streamlined shape. No air would get caught behind the gate.
But that is not the case. I have it on the authority of the Mythbusters that driving with the gate down uses more gas. How did they determine this? They drove two pick up trucks: one with the gate up and one with the gate down. Both had the same amount of gas and the one with the gate up went further. About 30 miles further on the same tank of gas.
It seems that having the gate up causes some air to stay in the bed and work to make a better streamlined shape than with the gate down. With the gate down, more air pushes on the bed of the truck thus causing more drag.
Thanks Myth Busters!
