I had 235/40/18's on the front of my lowered car and it rubbed a little bit in several places but it didn't hurt anything. So you have 5 mm on the inside with the stock rim and offset. Then I put a 10 mm spacer with a 235/40/18 and they fit but polished the fender liner bolts a bit, but nothing serious. So there is a maximum of 5 mm on the inside and 15 mm on the outside with the stock offset. So a 45 will work with the stock tire size with no rubbing on a Crossfire. As you know, I'm on 245/35/18's on 18x9 rims with a 30mm offset and NO RUBBING ANYWHERE! With this info you should be able to figure out what fits on the Crossfire, I don't have a clue about a SLK.

Les