And the problem is, the people who want to try a thing as stupid as putting a car tire on a motorcycle, for some reason cannot be convinced to not do it...
I've put about 135,000 miles on two different Goldwings. Yes, I know. That's more than some people, less than others.
But my point, I rode a Goldwing that was the same model as my current one, with a car tire on the back. After about three miles, I was ready to pull it off onto the shoulder, and let the owner come and get it.
It also makes me wonder, if there was an accident involving a motorcycle with such an obviously unapproved piece of equipment, if the insurance company would resist in paying on the claim, and cite negligence by the owner.
Don't think that an insurance company wouldn't jump at the chance of getting out of paying a claim.