Based on the Airtab instructions the VGs should be placed as close as possible to the rear trailing edge and have a specific side spacing. Here's an excerpt from the instructions:
I could probably stand to move mine further rearward. In your case, I would recommend you try again if you still have the VGs and place them in a different pattern more inline with the instructions.Airtabs™ are installed as close as possible to the rear (trailing) edges of the vehicle and with the wide
leading edge vertical. If the Airtabs must be moved forward to avoid rivet heads, obstacles, awning bars
etc., keep the wide ends of the Airtabs within 12-18 inches of the back for optimum performance.
Space Airtabs no closer than 4 inches
(10cm) on center. Increase spacing to avoid rivet heads, lights, grab bars etc. but do NOT decrease spacing as this will
decrease performance. Ensure that they are mounted in a straight vertical line. “Staggering” of the Airtabs to
accommodate body curvature is fine as long as the spacing remains consistent.
Airtabs™ can be spaced farther apart than the recommended 4 inches on center to avoid ribs, rivets, running lights, grab bars, back up lights etc. They should not be placed closer together because the adjacent vortices will interfere with each other and reduce performance. As much as we'd like to sell more, more in this case is not better, and 'shoulder to shoulder' mounting is counterproductive.
Lastly here is a test of vortex generators placed on a Honda Civic hatchback tested in a Lockheed Martin wind tunnel (spoiler: they work!)
http://www.airtab.com/Site/documents/Ho ... portV6.pdf