OEM winter set is actually 18x8 et20 - 235/40/18 all around. I don't have an OEM set for winter but my aftermarket set is the same spec and people might bag on your for running thin tires or whatever but 2 things to remember; 1) safety FIRST. 2) you're driving an M3.
Current: 2013 Fire Orange E92 M w/orange stitching
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