it seems that on recent cars the bumpers have gotten a little wider down low, but most still leave a little bit of the tire side exposed neat the bottom end.
one car wich has an interesting feature and some pictures on this area is the tesla roadster...
in the first pic there's a virtual model wich shows the airspeed accross the tire edge
in the second pictures there a mock up wich has "reverse mudflaps" taped to cover this area, and when you look carfully the production model seems to have retained this feature
now this could very well be a stability air rather than drag reduction, but a similar setup might benefit existing cars with the same configuration...