Recommendation from dealer here is to use the 255s as "max" at rear and also 255s as "min" in front. Thus -> wheels rather 640M than using the standard summer ones. 80% good info and Maybe 20% bmw marketing wanting to sell their wheels. But so far am happy with their previous recommendations. I should mention that we actually get snow here
, elevation 1200-6000ft within 1 hr drive.