Just saw on Mythbusters where they disproved the notion that driving with a truck tailgate down improves gas mileage. They showed how with the tailgate up there is a rolling "ball" of air right behind the cab that deflects the oncoming air up and over the rear of the truck bed. With the tailgate down the oncoming air falls rapidly right over the cab into the bed of the truck creating LOTS of drag. Who'd a thunk it??