I was looking at some 15" N90's for my '78 4-speed car to use for road trips. It came with the 14" Rally's, which I would keep for shows, and for originality's sake. It's just a little too wound up on the highway with the 14's, and I felt like the 15's would help with that.. A local guy has four Monte Carlo SS wheels that he says are 15". Is there a difference in offset, or other differences I should be aware of? I don't really care if the casting numbers are different, these wouldn't be original to my car anyways.. Any thoughts? thanks, guys!