On the mythbusters tailgate thing. It is conclusive that tailgate up gets better mileage. This is for the same reason that in your average rainfall you can drive and not get anything wet in your bed. There is a bubble that is created that causes a very nice constant flow of air that falls aboce the tail gate. With the tailgate down you actually end up with a pressure on the tailgate that pushes down increasing the force and effectively increasing the load.
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