And if you want to go with 18's, I have a set of OEM X3 sport package 18's available from a 2008 that were only used for 1 year and look brand new. they will clear the 4.8is brakes, but you will need to rebalance with outside weights instead of where the factory puts them. I will sell them for $450 plus shipping from CA if anyone is interested. PM me for pics. then you can go with the stock non sport 18" tire size which I think is a 255-55-18. That is a pretty ideal snow tire size, IMHO.

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2003 4.6is - SOLD
2003 M5 Imola
2008 Mini Cooper S
2011 Mazda CX9