I was thinking of a Zzipper or similar front fairing only. There is drag from the front wheels. Seems that the cleaner shape is worth the extra frontal area.
Wheel disks allow smooth air flow, at least when pointed straight ahead.
My own tests with a Zipper vs. a large cardboard nose cone seemed to bear this out. They seem too big for a roof rack. Any thoughts would be appreciated. I built a roof rack which has 3 channels for the wheels, and holes you put pins through to lock them in place. It swings down in the manner of those wheelchair roof racks.