225/55/15 should fit fine without any major problems. All you might have to do is roll the rear fenders using a wooden baseball bat and a rag. I personally run a staggered setup that has greatly increased the "fun" factor and ultimate grip of the car. I run 205/55/15 Bridgestone Potenza RE71's in front (VERY STICKY, only $68 a piece at tirerack) and 225/50/15 Sumitomo HTR50H's (a bit less sticky that RE71's, but very progressive breakaway for easy to control slides, only $65 a piece at tirerack). Only problem with this setup is that the tires, especially the RE71's, have very stiff sidewalls, which degrade the ride quite a bit. But if ride doesn't matter, this is a damn fun setup for a 535i.