What you could do is use 225/45/18 and 255/40/18 with those 18x8 et34 and 18x8.5 et37. It's fine for xdrive the diameter difference is only 0.22% and since you aren't planning to lower the car i doubt there is going to be any rubbing issues. Here is a nice comparison with style 162 and stock vs. upsized tyres
http://www.e90post.com/forums/showth...2#post14323772
Just remember those tyre sizes affect your speedo making it showing
slower speed than you really are going, but in reality BMW's Speedo shows
more than true speed from factory anyway. So it would be good idea to check with gps what's the true speed really is.