Andy, depending on the conditions when you get to lake tahoe, the roads may be totally clean and you may be all ok even with summer tires.

But... I moved to SF bay area from Ontario, Canada back in 2000. While in Canada, always drove on 4 season tires with no issues in snow conditions. In my 2000 Toyota Solara with 4 season tires on it, I drove to Tahoe and got stuck in the driveway at the first sprinkle of snow... I couldn't believe it... what kind of 4 seasons tires were these on my Solara anyway?

For the X5, I got Michellin Latitude Tour HP as they are designed (apparently) to do light snow as well. If I had any type of 'performance' tires on mine, I would not want to take it to a snow condition, as the hassle (and embarassment) of being stuck in a light snow especially in a 4x4 would not be something I want to deal with.

I suggest a second set of wheels with some better snow tires.

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Currently driving: 2011 E70 x35i , 2011 E90 335d
Previous BMWs: 2007 E90 328i , 2005 E53 3.0i, 2011 F10 535i (6spd manual)