[aerohead]

This cropped up during the time spent with SAE Paper 2018-04-03. On a CdA basis:
1) the 5" airdam demonstrated a 4.14% drag increase with rotating wheels.
2) the 6" airdam demonstrated a 3.4% increase.
3) the 6.35", a 2.61% increase.
4) the 7", a 1.57% increase.
5) the 8" demonstrated a 2.3% drag 'reduction.'
6) the 9" demonstrated a 5.65% drag reduction.
This would constitute one example in which the effects of a rotating-wheel wind tunnel investigation could not be predicted, nor expected in advance of testing.
I believe MIRA or Coventry University experienced similar phenomena.
7) NASCAR race cars and Bonneville land speed record cars might provide some interesting quanta in light of the near-ground-contact airdams.