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I would think that the difference of hub cap design would depend on the vehicle in question. Has anybody seen tuft testing done for stock vs disc on a particular vehicle? Has anybody seen windtunnel tests at highway speeds for the same?
Using bicycle wheels as an example (not quite the same since bikes have no fenders, but stick with me here)
High end Disc wheel: Zipp Sub9 @ 1000g
High end Spoked wheel: Mavic R-Sys @ 785g (rear)
The heavier Zipp is far more aero than the Mavic and can shave several minutes (~4) off a 100 mile course vs the lighter spoked wheel @ < 30 mph average speed on the same bike. More aero @ speed = < power output = energy saved
So, since the aero benfit can be had (negligible as it may be) at a seriously minimal cost, why not? Even if the gains are so tiny as to be nearly immeasurable tank to tank, over the life of the car it may pay for itself.
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