255/40 would be the way to go for staggered rear 18's
235/35R19 would be an option for 19" fronts. That would be the equivalent dimensions of the BMW recommended 235/40R18 winter size.
To credit the forum, click through one of the banners in my signature and the top of the page, or call me direct at 877.522.8473 ext 660. Email:firstname.lastname@example.org