Because people will ask for it, I went and found the cited airtab study (I think this is the one):
[Anon,] "LOCKHEED GEORGIA LOWSPEED WIND TUNNEL HONDA CIVIC HATCHBACK AIRTAB(R) MODIFICATION RESULTS"
http://www.airtab.com/test-results.htm
"Executive Summary:
This report shows conclusively that the Airtab® product reduced aerodynamic drag
forces at the base area (the rear facing surface) of the test vehicle. The test showed a 4%
reduction in horse power required to maintain a steady speed of 55 mph. Only the sides
of the vehicle were fitted to assure the most aerodynamically symmetrical run data. By
adding Airtabs™ to the rear roof line as well, a conservative performance benefit
extrapolation of these findings would be in the 50% range resulting in a further HP
required reduction to approximately 6%. The test also shows that the vehicle drag
coefficient (CD) is reduced at every yaw angle from zero to thirty degrees angle from the
airflow and that this CD reduction increases at greater yaw angles."
