One challenge of using hub motors in cars is what to do with the mechanical brakes. They're required by law, even if electromagnetic braking (regen) can be as effective.
See how this problem was overcome with this hub motor driven pickup:
http://ecomodder.com/forum/showthrea...50-a-5914.html
(disc brakes on the drive shaft)
Another problem is that hub motors large/powerful enough for automotive applications are new, so expensive & relatively rare.
And here's a retrofit kit that doesn't replace the wheel with a hub motor, but adds one to the existing wheel:
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