I was walking out of a store when I noticed my rear tire looked a little funny. The scallops can only be seen when a certain part of the tire is exposed. The rest of the tire seems fine.
Scalloping or cupping of a rear tire is almost always underinflation. If it only goes part way around the tire, it is probably out of balance.
As others have said, running a car tire on a motorcycle is a really bad idea. The construction is very different because the forces exerted by a motorcycle are totally different than the forces a car demands. I have heard people say things like, it's just a cruiser - i'm not dragging my knee around corners. Okay, but you're still leaning the bike at anything over parking-lot speeds. That's a force that the car tire is not meant to cope with. I have heard, my friend did it for 60,000 miles, and it was fine. Your friend got lucky. Motorcycles rely on 2 tires to provide traction. If 1 fails, you're in a LOT more trouble than if 1 fails on a car. Why risk your life to save a few dollars? a decent set of motorcycle tires is under $200. How much is it going to cost to repair you and your bike if the car tire fails?
First thought in my head was an out of balance condition. I have seen this on front tires, but very rare on the rear.
