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I always thought the this was wrong. Tailgate down should be more fuel efficient. I always thought that the tailgate up would act as a big flat panel stopping the air stream. so tailgate down would lessen that drag.
I have learning a lot about aero in the past couple of months in this forum. Most of the drag in the rear of the car comes from back pressure. The more flat surfaces that are perpenducular to the airstream create low pressure areas and thus create drag.
The pickup bed up gives one step at about the correct angle for limiting the drag. With the tailgate down you get the full effect of the entire flat surface of the back of the cab. This creates a large low pressure area. The tailgate up lessens this some.
Thus tailgate up should be better.
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