I'm fairly certain that BMW states that chains are to be used on rear tires only. As far as the use of 20" summer tires... I have no experience with Lake Tahoe, but I would expect you could encounter a variety of winter road conditions, from slush to packed snow and ice. It is the packed snow and ice that I would be concerned about given your use of summer tires. Your steering response and braking dynamics will be severely comprised. Driver skill can go a long way to overcoming some of this, but your fellow drivers will have a lot of influence, and sudden steering or braking inputs may not be possible. I have read other posts from members who have talked about using all season 20" tires, but don't recall a summer tire on snow post. If you don't already have experience driving in snow, 20" summer tires on possible challenging roads conditions may not be the best place to start learning. If you are experienced, road conditions and the possible advice of other members may help you in your decision. I run dedicated winter tires each year, and while I could get by on the stock (18" in this case) tires, it would certainly make it much more tricker than it can already be. Good Luck, and if you decide to go with your X and 20" with chains, let us know how it goes (who knows, maybe the weather will cooperate and grace you with clear roads, but as you are already planning for... be prepared!