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VGs do not keep airflow pasted to the roofline. Thats just not what they do. They will induce turbulence (which is bad) to slow the air down and allow it to follow a steeper curve. This, at the very best, increases the pressure on the rear window of a sedan which does help. But, the turbulence induced by the VGs normally offsets any gain.
I have never seen any documentation (from people not trying to sell VGs) that shows that VGs improve mileage. MetroMPG has also done testing that supports this.
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