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10% = 5%
a search for Glen Scharpf, aerodynamics engineer for General Motors, landed on an article, 'AHEAD OF THE CURVE,' in the Chicago Tribune, February 12, 1998.
In the article, Glen attributed a 10% change in aerodynamic drag with a 5% improvement in fuel economy, at 55-mph.
This is what he used with respect to the Honda CRX-HF aero-modding data from CAR and DRIVER and Bonneville to come up with Cd 0.235, back in 1991.
It's not relevant to any EPA HWY test cycle, just constant-speed 55-mph cruising.
It may just be some internal metric specific to GM's Aerodynamics Laboratory.
So if you've seen me use it in the past, this is its origin.
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Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/
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