I don't think downsizing from 18-17"s for snow tires is necessary for NJ winters. True that 17" snow tires are cheaper but that price difference would be offset by the cost for an additional set of wheels. I would just get some Blizzak LM60 snow tires for your 18"s and you should be good to go. 17"s just looks really bad on the F10 and I couldn't bare to look at it that way for 4 months of the year but thats just me. Getting narrower tires will have a bigger impact for snow traction than going from 18 to 17"s

Alan

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Current - 2013 N55 535xi Msport SAT
Retired - 2009 N54 335i Msport 6MT
Retired - 2000 M52TU 323i ASA Supercharged 5MT
More photos of my car here