Hi Donee -
Mitsubishi measured the boundary layer thickness at the trailing edge of the Evo roof at 30 mm - just over 1 inch, which is close to the height of the AirTabs (1 inch = 25.4 mm).
In testing different VG heights, Mistubishi says:
Quote:
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...the drag reduction effects for the VGs of three different heights (15 mm, 20 mm and 25 mm) were all equivalent to –0.006.
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That's not to say the AirTab is the ideal shape though. Again going back to the Mitsu example, they specifically chose a VG style (delta-wing shape) because it had a small frontal area. The AirTabs seem to have quite a large frontal area (even considering they create a pair of vortices), and that may be part of the reason why they didn't reduce overall drag & show any fuel economy improvement.
chuckm - an AirTabs distributor sent some to me.