Yokohama Tires added more 'spoilers' on tire sidewalls. The tiremaker molded angled fins on the side walls to help reduce drag while the tires are spinning. The upper part of the tire side reduces aerodynamic drag while the lower side fins help reduce lift.
How about using this research and ecomod test it on your 'smooth' wheel covers?
News Release | Yokohama Rubber Advancing Tire Aerodynamics Technology