One thing i can say is, even without the 5-10mph headwinds on the drive there. Coming back with a tail wind, the car still felt "heavy"... The airdam, may have caused this, but sure as hell, it may not gain speed as fast downhill, it does definitively coast much further!
Overall for about an hours worth of work, and 10mins at the hardware store, im pleased with the results.
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It sounds like your air dam is producing downforce, creates more traction, but more RR.
I had a very similar setup on my Towncar, it made it very stable in crosswinds
