Originally Posted by jurrian @ eas
18MM for the rear wheels would probably cause some rubbing issues. I would say 15mm at VERY most, and even that causes very slight rubbing. If you don't want rubbing, I usually suggest 15mm up front and 12mm in the back for the best look without rubbing.
I just had the the 18-15 setup installed on my M3 E90 last week. If I was told I would have rubbing with 15mm in the rear, I don't think I would have gone that aggressive. I hope I don't have issues and possible damage.