Found newer wheel study on Volvo. Some interesting things there.
http://publications.lib.chalmers.se/...588/255588.pdf
Difference for best and worst design is cd 0,02 which is massive. On general with smooth hubcab you can lower your drag coefficient about 0,01.
Convex mooneye hubcap will lower the drag about 0,027 so to have them convex helps litlle bit more.
I am at the moment thinking about to make my own aerodynamic wheelcover model for steel wheels with traditionel fastening mechanism.