I was at the Mid-America Truck Show in Louisville,KY yesterday and was pleased to see various aeromods (intended for semi trucks). I was particulalrly pleased to see at least 3 different types of sero wheel covers.
One design that I was particularly impressed by was by a company called "Flow Below":
The covers can be purchased separately or as part of a kit that includes additional fairings:
What I loved about the design is the "snap on/off" feature. To remove the cover you simply push the button in the ceter and twist slightly and it pops right off. To re-install, it simply pops back on. Simple and easy.
I talked the the gentlemen at the exhibit (who I believe was the owner and inventor). He said the cost (for the whole aero kit) was $2500 and 3rd party testing had verified fuel savings of 2.2%.
The only sad news for us Ecomodders is that it's currently only made for 22.5" & 24" rims.
FYI, here's the company's website, which I'm afraid doesn't have a ton of detail:
Tractor Aerodynamics Products : FlowBelow