[aerohead]

I believe that the origin of the 10-to-5 rule can be attributed to Kent B. Kelly, and H. Holcombe, General Motors Corporation, SAE Paper No. 649A, June, 1963.
' a 10% reduction in aerodynamic drag makes a 5% reduction in fuel consumption possible at 55 mph, and a 6% reduction possible at 70 mph.'
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Curiously enough, forty-six years later, in his Fall, 2009, Masters Thesis, Feysal Ahmed Adem, California State University, recorded a bona fide 4.997% fuel economy improvement, for a 10% drag reduction, in his Dodge Ram 1500 pickup truck with 3-D, 12-degree aerocap.
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Even more weird, was the 40.7% rear lift reduction from this 'fastback', modification, compared to the 17.5% rear lift reduction Texas Tech indicated for a 'notchback', full-tonneau cover.
Isn't it interesting?