Just wondering... are those rings absolutely necessary? I ask because I know someone who has these old aftermarket wheels on an old Escort and the centercaps are all missing so I can clearly see that there are no rings on it but he says he's never had an issue with vibration or anything with it like that. Just wondering how important they really are and if it varies from car to car.
Edit: Well I've found one source that says even if it's centered by the lugs, the studs have more stress put on them because normally, the wheels actually sit on the hub, not the lugs. If that's the case, I'd want metal rings, not plastic.

I really have no idea.