I'm no expert on this, just spouting off my opinions. You live in San Francisco and want drive in the mountains occasionally. I doubt it gets subzero there, but I'm not sure. You will probably be fine with the 18s. I live in Minneapolis where temps can get far below 0F in January and of course it snows. Below zero, ice builds up on wheels much more than 10-32F. I am planning on needing winter tires from November to April. In other parts of the country people can probably get away with 18s if it just dips below freezing and have some slush occasionally. The 18 TSportonlines have documented fairly good (6.35mm) clearance for an 18 in their video on youtube. I'm just worried it's not enough where I live. Wheels with 3mm of clearance seem like a terrible idea in a Minnesota winter. Unfortunately, we are all new to this, since this car didn't exist last winter.