Upon further reflection, those little air dams might be for a venturi effect, because they only extend half way into the tread area, and have a slight L-shape. The air flow is forced around the tire to the brake side, breaking up the vortex of the spinning wheel.
There has been some discussion here on aerodynamics, Cd, downforce, so here goes:
On the Lexus LS 430 there are wheel spoilers, (air dams?) located on the undercarriage in front of each wheel well. They are made of synthetic rubber, are about 200mm wide and extend about 30mm down from the car...
