I went a little bit nuts on my X2 for winter.. I'm running a 255/35r19 with offset +25 It's starting to look like a lifted truck
The wheels themselves stick out maybe a tad outside the fender. The calculations put it 47.4mm wider. In terms of rubbing, it will never happen since the arches are so super high on the X1 (and X2). You could even probably make the patch even wider than a 255 but I would really see it only starting to hurt performance more. The winter tires to boot are already a nicer ride than the stock runflats, and definitely the extra width makes this thing stick to the road a lot more. I'm not out there to flog the car though