*Going 'long' instead of 'short',you're still separation free.
*You take a little hit on surface friction drag do to the added wetted area.
*And since the boundary later is thicker back there,its kinetic energy can't be recovered as base pressure rise once the flow does detach beyond the trailer.
*But you've got all that added useful cargo space.
*The area of the rear bulkhead,compared to the 29-sq-ft of original frontal area will give you a sense of your wake reduction.
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From a loose rule-of-thumb,by William H. Bettes of the GALCIT wind tunnel at Cal Tech; without the weight increase up to 5,000 -lbs,you could add about 7% to your MPG.
That 30-mpg you pulled around OKC would be over 32-mpg if not for the weight.
Almost a 69% improvement over the naked T-100.
