I have 255/60/18s (originally bought with wheels from a L322) on mine with no problems so far with fouling of the arches etc. They do tramline though but I don't know whether that's more a function of the make (Goodyears) than of the size. I believe from the forum that Pirelli ST's are much better in this respect. 18in tyres can be found pretty cheap on fleabay as everyone wants to 'upgrade' to 19s and 20s. At the time, a full set of brand new wheels and tyres was cheaper than 255/55/18 tyres on their own. Probably even cheaper now.