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I think they create a vortex, and that helps to prevent the buffeting that would otherwise happen with the window open.
Typically air rolls over the A pillar from the bottom of the windscreen to the sides, and that would then hit the wing on the mirror and be guided upwards.
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2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
  lifetime FE over 0.2 Gmeter or 0.13 Mmile.
For confirmation go to people just like you.
For education go to people unlike yourself.
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