I can say this. I have winters on both cars and believe they are much safer than anyone on all seasons. Chicago conditions are much like NJ and I also have two children. I am not sure how much better the 18's would be over 19's In the snow because you would be driving more cautious and i am pretty sure you are more worried about the other drivers My concern is having to do an evasive manuver in dry or wet conditions and feel the 19" will perform better. Plus the excellent AWD that BMW has will just complement any snow tire.

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2012 X5 50i ZAP(2VA, 336, 465), ZTP, ZPP, ZPS(6VC), ZCW, ZRC, 322, 4UB, 4NC, 610
2011 ///M3 Silverstone II
2011 X5 50i(lemon)
-- SOLD --
08 550 Msport
07 X5 4.8I sport
04 545i sport
02 4.4i sport
01 540I M/sport
00 SL500
97 SL500