It's inventor is our inactive member Brett Herndon ( aka 'Bondo') , a master clay modeler, of Little Rock, Arkansas, creator ( and Patent-holder ) of "Aerolid' and 'AeroTail,' both wind tunnel tested.
I suppose Brett has a means to back up his claim, which may involve caveats, as aerodynamic appliances often do.
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The device IS a box-cavity, relieved at the bottom, to allow the tonneau cover to be opened and tilted, without colliding with 'Dragshield.'
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In the early 1990s, member AeroStealth fabricated a plywood device, very similar, of which I participated in flow and mpg analysis.
That dog 'll hunt!