Well not "anything over" 3". 4" would be optimum (although I suppose 3.5" would work too if that size is available). Not sure what your last comment regarding "swallowing" the original tip refers to, but the larger tip replaces the old one, it doesn't go over it.
Thank you very much paris1. Ok, I think I have misunderstood the whole idea... Do you mean that I am supposed to take out the original tips and then install the new ones? I thought that the new ones would just be pushed over the original ones and then tighten the screws/bolts. I think I'll go and take a peek under my car thanks again, have a nice day!