I'm all for properly matched wheels and tires for the conditions you will be driving in. That being said, here in Indianapolis, it dropped below freezing and snowed on 12/1 and we've had intermittent snows weekly since then with side street coverage and ice here for the last month. Our snows have been about 2-4 inches each time so not A LOT by any means, but enough to make the roads unsafe for sure. Because it got cold so early, I didn't have a chance to swap out my 20" wheels with the Dunlop Sport RFT's on them for the 18" set with all season tires that are sitting in the garage. (Not motivated enough to change out tires in the cold just yet). I have been pleasantly surprised with how well the car tracks in snow measuring 4" or less with these summer rated tires and with 315 width tires in the rear. The BMW handles, accelerates, and stops much better than my wife's Toyota Solara FWD vehicle with brand new all season radials (less than 500 miles on them). I'm not saying you don't need winter tires and undoubtedly they will improve wet and snow traction in the cold but just wanted to share that if you live in a relatively flat environment with a few inches of snow, the X5 can be driven safely and comfortably in these conditions even with the 20" wheels and tires on.