I bought a set of the 20" #214 (off a X6) for my X5M for winter. Tirerack recommends 275/40/20 all around (even on the 11" rear rim). Either Blizzak or Alpines. Both options are about $210/tire.

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2013 Cayenne GTS Basalt Black/Alcantara (Wife's Daily Driver)
2013 Porsche 911 cab Aqua over pebble and agate (My daily driver)
2010 Porsche GT3RS Black Grey/Black Alcantara (Track car)
2012 Porsche CaymanR Peridot Green with carbon sport buckets (Wife's Track car)