I recently came across this paper:
Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars (Alexey Vdovin, 2013) which draws the interesting conclusion that in some cases, "ventilation resistance" (fluid forces due to the rotation of the wheel and the pressure differential between the wheel well and the side of the car, as opposed to fluid forces due to the overall motion of the car) may have a sufficiently large effect on the energy use of a wheel that a flat wheel cover may not actually be the best solution overall, despite having nicer airflow from an 'overall motion of the car' perspective.
The solution which appeared to give the best results (under the particular parameters tested) was a cover over the outer radius of the wheel while leaving small openings near the hub (see pg. 20, figure 17 (f)):
While pondering how this scheme could best be applied to my mags, I happened to park next to a Prius which had these wheels:
- apparently from the 2004 model. The 'trim' on this wheel looks suspiciously similar in general layout to the suggested design from the above paper. I can't find any reference to this being an efficiency feature, but maybe Toyota knew something about this that it wasn't letting on?
I'm thinking there would be two ways to fabricate something like this:
(1) As a ring of material ala the existing style of smooth covers seen here, with appropriate holes cut out, and zip-tied or similar to a mag wheel.
(2) As a set of 'gap fillers' which would fit between the spokes of a mag.
I don't think this is likely to work with covers on pressed steel wheels though, as the existing holes are generally nearer the rim than the above examples...