Wouldn't
255/40R21
295/35R21
be a better fit?
https://tiresize.com/calculator/
shows 0.3% variance with this setup.
Whereas with 295/40R21 there is a 4.5% variance.
I love fat tires in the rear...
There's an X3 M in my parking garage this week and I really like the overall look of 255/265. Nice and beefy all around, though the rear could be beefier in my opinion.