What are these things?

On some tractor trailers, we've seen wheel inserts. I got a picture of some... posting on my blog so Jimmy can tell me what they are!  I'll come back and update this post once he enlightens me!

Update: Jimmy called me last night. These are added to the wheels as an energy saving mechanism, supposedly making the air slip more easily past the wheel. There is probably science to indicate under what circumstances it is more or less effective (I would say, "the jury is still out!").