[woodstock74]

What's the consensus, is it the reduction of the volume of air that passes through them that is cut off/reduced (i.e., closing up the spokes and sealing them), or that they are flat (which does the same of course, but with the added benefit that they are flush to the tire) that is responsible for the drag the reduction?

I'm contemplating modifying my existing hubcaps by simply filling in the "spokes" and insuring no gaps between the hubcap and the tire, but this won't make them flush.