Originally Posted by STi_traitor
This is just from what I have heard and what I am running now. 235/40, I don't have the ZCP package but I don't think that makes a difference. The reason behind the 235 is that it helps to cut through the snow with the thinner tire. Looks stretched like crazy in the rears but I figure it is recommended and the safer bet to keep my car in one piece for next summer. I have heavy snow days, not sure how your weather gets.
Check your driver side door, I believe it has the recommended sizes with PSI.
BMW actually recommends 235/40/18 in the front and 255/40/18 in the rear (on their winter 18x8, 18x9 wheels). They don't recommend 235/40/18 on the 9.5" rear wheel. That tire is too narrow for a 9.5" wheel.