The sidewall would be exactly the same with a 17 if you are comparing a 315/70/17 vs a 315/70/18. The sidewall = 315*70% = 220.5mm in either case.
What I was saying is that the offset I am using is a +30 vs the +55 factory Jsharifi was talking about. Also I am using a 17 inch wheel with 315/70/17. That equals a total diameter of 872.8mm (34.36inch). If you had the same tire on an 18 wheel (ie a 315/70/18) that would be an 898.2mm (35.36 inch) overall diameter. I am saying that size would probably not fit.
I also don't know if there is such a thing as a tire in 315/70/18. or if there is an 18 inch wheel available with a +30mm offset.