With reference to the German BMW forum (1erforum.de) and the findings of one of the forum fellows, I did a test with my recently mounted Michelin Pilot Alpin4 tires, front 235/40/18, rear 255/35/18
That fellow pretends that, accelerating from 140 to 180 km/h, DTC light is blinking all the time with instability as a consequence. He let the rear tires change to 255/40/18 and way the problems.
I did the test and indeed he is perfectly right. Only difference, no blinking DTC light.
My findings : vibrations on the rear axle who increase with speed. In my opinion, the stability systems indeed intervene and try to hold the rear wheels on track . Strange and scaring feeling, the car becomes less stable, but no lights blinking at all. Anyway the behaviour is completely different compared to the summer set. I do not understand BMW is not yet aware of that.
My rear tires don't have the BMW "star identification" while the front tires do. Tires have production week 35/16 (Aug/Sept).
On the other hand it seems productions from November, do have the "star identification". But what will be the difference in behaviour ???
What I know and what I think is that German test M2's, the ones we all saw circulating during last winter, had 235/40/18's all around. Didn't they test the 255/35/18's on the rear???
Anyway, during winter, accelerating from 140 to 180 km/h seems to be rather exceptional but it should be possible without those severe problems.
Comparing summer set with winterset :
245/35/19 - 265/35/19 : rear = 14mm taller than front
235/40/18 - 255/35/18 : rear = 10mm shorter than front
With 255/40/18 in the rear, rear is 16mm taller than front. Wouldn't that be the better choice ?