truck tailgate

Take the truck tailgates and have a runner on each side hidden in the body, so that the tailgate can be lifted like a garage door and lock in an upward, flat position.
Traveling at moderate speeds, this would eliminate the drag the tailgate causes and at high speed act as a spoiler to give the rear end additional traction.  In either instance, the upward position would be a gas saver.   When needed for loads, the door can be unlocked from the upward position and lowered.