tire diammeter of stock size tire on 18 vs 17 is 1/4in larger. There is no difference on rubbing between 17 and 18, it all depends on the tire brand, all brands have different sidewalls. I had 18 in Pzeros and did not rub much, with Eagle F1 the side wall is sticking out much more and they rub like mofo( the dont anymore 5mm spacer took away most of the rubbing)
Re: I have 17", but will the 18" clearance be ok (kitkat)
Each Volvo is a bit different. MOST rub with either size wheel. This also depends on the brand of tire and the width of the tread section of different brands of tires, it's that close in there! 5mm spacers usually cures the problem with all.