That's a tough one, as "common sense" leans towards saving fuel but I know Mythbusters done something on it and the results were surprising!
The myth busters one was about driving with the tail gate up or the tail gate down. Logic says tail gate down is more efficient but the MB's found that the tail gate up produces a vortex or rolling air in the tray which acts like the roof of a sedan. Drop the tail gate and this vortex is lost resulting in a lot of turbulant air behind the vehicle.
Cheers
Bennie
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