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For anyone else who got the 21" wheels... I had first experience with snow on this weekend in Model S. I had been expecting it to handle at least slightly better than a "regular" car based on other reports that I had read online about Model S being very good in snow because of unusually low center of gravity etc.
Well... it was very unpleasant :/ No winter tires, just the regular performance tires. We were going ok on the flat areas in the unexpected snow. Attempted to go up a small hill and Model S dropped down to only a few km/h speed because of traction control and could hardly move at all. In the end had to carefully turn around and go 200 meters or so back down the hill, but brakes would not work well at all. Very strange, jerky behaviour, where the car would go from about 2 km/h to suddenly 8 km/h or so then slow up again. Even with fairly strong brake pressure, Model S couldn't come to a complete stop, so it was very lucky nothing was in front of us until we were safely back on the flat area.
So... my advice has to be, don't attempt to drive Model S in the snow with the 21" wheels. If you find it snowing unexpectedly, like we did, better to just park and wait or figure out some other way to get to where you need to be.