[12v_dub]

I have 20k miles on my m40i which came with 21" wheel option and need new set of tires pretty badly as the rears are worn all the way down to the low indication mark.
I want to ditch the Run flats all together for better and smoother ride and go with a new set from Michelin Pilot Sport 4S. The only issue is the fact they don't have a front set 245/40/21 but they do offer 255/40/21 which only add about .3" in height increase when comparing the rears ( see photo below) I spoke to a Michelin rep few months back and himself doesn't know when a new option will be added to their 21" menu. I live in the south so I don't have to worry about snow, all seasons were also being considered but there are o options in 21" size.

[sinbad4]

Bumping by 10 on width shouldn’t be an issue. Worst case, get the 15mm spacers, guaranteed no issue then and you might appreciate the slightly wider stance.

I agree about losing the run flats. I intend the same.

Call tire rack, if in doubt. They know.

[Replicant]

I don't think you are supposed to go over 1% with xdrive. The stock is .3% off.

You could do 285\35\21 & 255\40\R21 and maintain the same ratio though. Though they don't make the PS4S in the 285 size, only PSS. And they don't make PSS in the 255 size. I really hate these wheel sometimes. Nice to hear you got 20k out of them though.