A couple key findings on smooth wheel covers of interest to this forum, from:
Sofie Koitrand, Adrian Gaylard, and Gianluca Orso Fiet, "An Investigation of Wheel Aerodynamic Effects for a Saloon Car" - Conference Paper - September 2015
Conference: Progress in Vehicle Aerodynamics and Thermal Management: Proceedings of the 10th FKFS-Conference, At Stuttgart, Germany
The PDF is currently available online for free. Thanks to the authors and their professional org for that. I will share just a small portion of their findings, but there is more. Go get their paper and read it if you want more.
In another thread, a few forum members have been discussing the effectiveness of several aeromods for wheels. This study uses a wind tunnel (with 5 belts) and CFD to study these different wheel designs:
The conclusion was that the fully blanked, smooth cover wheel saw the greatest reduction in drag:
That graph presents a combination of the wind tunnel & CFD data. In the wind tunnel alone, the fully blanked wheel saw a delta of -0.013 Cd, the 85mm annular blanked wheel -0.009, and the 430mm center-blanked ("Tesla" style) wheel saw a -0.003 Cd delta.
Their conclusion is that in this test
fully smooth covered or blanked wheels produced the greatest Cd benefit over baseline, whether measured in a wind tunnel or using CFD. This is a different conclusion than some other recent studies.