I don't think there's any easy way to tell you what the drag of
your wheels are vs smoothies.
I didn't crack open Hucho to see what he had to say on the matter (I'm sure he's got stuff about it), but from the PDF
Contribution of different devices to the total drag (Road Vehicle Aerodynamic Design, Nédélec-Berroche, Neiter, Rousseau, Suard)...
Quote:
Effect of rim type:
There are two main types of rims. First, the rims which are entirely full, i.e. without any discontinuity or gap in the surface (see below the example of Citroën Bx rims) are the best choice for reducing the drag coefficient. Indeed, if the rims are full, there is no possibility for the air to go threw the wheels and so, it doesn’t disturb the flow under the body of the vehicle and on its side. But this type of rims has an important disadvantage: it would not enable an effective brake cooling which becomes very important when dealing with performance-oriented cars. That is one reason why the car manufacturers have introduced some rims with spokes which enable a certain air rate to flow threw the wheels and hence to cool the brakes. Of course, this type of rims has also often a good impact on styling but there is no denying that it is drag-consuming. Indeed by completely sealing all four rims, Cd can be reduced by 0.08 to 0.012 (ref. 6). The maximum drag difference between a good and a poor set of rims is probably even larger.
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(I think the .08 is a typo and should be .008.)
Unfortunately it doesn't give examples of the rim types they're comparing to smooth covers, but .008 to .012 improvement is nothing to sneeze at.