[COcyclist]

Thank you all for responding. I think we have had some good discussion. I would recommend that if you are reading this thread, in the Aerodynamics forum, on Ecomodder, that you buy the book by Julian Edgar and decide for yourself. (I have never met Julian and do not stand to profit in any way from this recommendation) I agree that Moon style wheel covers are best for smooth airflow and low drag, in isolation, as proven in the wind tunnel. In the real world with spinning wheels and tires, a moving road surface, disk brakes inside the wheel and complex airflow around and under the vehicle, it becomes much more difficult to say with certainty.

Several studies are linked in this thread. The Chalmers study is on the web and graphics from it have been posted here. The others are SAE papers. IIRC they cost can $50 each which makes it harder for us to access them. Julian Edgar has purchased and read these and numerous other papers and studies. He worked with an aerodynamicist while writing his book. He has done countless hours of testing himself and shares his test results and test methods in the book. There are lots and lots of photos of aerodynamic vehicles (many shared by Ecomodder members) and color graphics. In the US the book is available on Amazon for $50 or about the cost of one SAE research paper.

In fairness, I did not test full Moon wheel covers. I have run taped factory covers in the winter. Eventually the tape came off and I did not see a measurable difference. (Once I did get snow buildup behind them that melted, refroze and threw the tire balance off enough to make the car almost undrivable). I have a bumper to bumper belly pan with no underbody vents and the upper grill blocked. Perhaps this causes more cooling air to be vented through the front wheels than on some other cars. I feel that based on the research, I may benefit from smooth covers on the rear wheels only. Perhaps that is what I should test next, if it ever quits snowing in Colorado and I can put the summer tires back on.