*evidently,their exists a force vector across the closed tailgate associated with the low pressure 'centroid' of the vortex which is common to the atmosphere exposed in the open configuration.This differential has been attributed to some of the drag reduction which is not present when a bulkhead is placed between the gate and leading edge of the tonneau.An inspection of GM's patent drag curve might quantify the difference.Whether the affect is major or minor is not emphasized in the literature.
*It would be good to have an illustration of what you mean by box.This could be interpreted many ways.
*A 'blister'-half-tonneau has been tested by myself and John Gilkison on the 1994 Toyota T-100.It demonstrated higher mpg than without it.
