I drove the TTRS through our last two winters with the Hakka R2 on them. That car normally wears 255/35R19 all around; I downsized it to 225/40R18. Had zero problems... but it has AWD, although no limited slip diffs (both front and rear are open). Roughly the same power / torque figures as the M2 though. Admittedly, the last two years felt pretty mild compared to stuff we've had in the past.
So I already own a set of (used, but at least one or two winters left) very good snow tires... I just want to make them work with the BMW, and out of necessity it will end up being a squared config. I ran the math on the diameters; my squared setup will actually be
closer to the stock rolling diameter of the rears than BMW's suggested setup!
