I also have the 17 inch spare in my 04 4.4, I hav the sport pack so in summer I have 19 inch Diamaris and in winter I run 18 inch Pirelli Scorpions. I find that they are almost as quiet and the ride is a little softer.

Generally, in up to about 4 inches of snow I like a narrower tire because it is less likely to sit on top - it will actually spin down and get to pavement. With all the traction aids on the X5, if you want to do that you have to shut off the DSC, knowing that I had X-drive I figured that it was ok to take the wider tires - imho they look better

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07 X5 E70 4.8i
04 Z4 E85 3.0