THIS IS MY PERSONAL EXPERIENCE AND OPINION:
I have 20" M-Sport wheels and the low profile tires are a pain in the a$$. I drive daily on very bad gravel/dirt roads as well as on sand/mud/curbs from time to time and my tires wear out so fast. Thats not all tho: if you drive the X on gravel(etc.) for too long the tires get worn out uneven and it screws up your suspension, active steering, and all that. It took my dealer more than a week(and a set of new tires) to fix the problem and over 10 failed test drives.

Also, depending on the road, my rims get damaged on many roads leaving scratches and chips all over the wheels.

With that said, I would still keep my 20" or the 215 21" and have them repaired every now and then, rather than 19" wheels.

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2011 ///X5 M (On Order)

2009 X5 4.8 M Sport Space Gray/Black Fully Loaded

2007 GMC Yukon XL Denali Loaded and Customised