I would think off or down would be better. Wind will hit the tail gate and cause presure in the bed of the truck and would cause the the box that you are turning to move down the track to go slow because of the wind that will build up in the bed of the truck. Hows that for a load of BS. I don't know how much air would get cought in the bed to slow you down but I would think some would. My .02 cents. Try it and find out what is better. I just heard less wind restriction with tailgate up.