[NTTY]

Thank you very much for your contribution.

Theoretically, we should keep the same offset which indeed leaves little room for wider rims. But having a 10mm variance is also reasonable.

Anyway, from what you measured, 9ET30 looks like a very reasonable choice and it's 1" wider than stock rims. Not a drastic change, but a change.

Rear obviously offers more possibilities, but since we have to keep the same rotating diameter front/rear, there aren't much choice in terms of tires. 1% difference is ok though (that's the stock difference between front and rear).

If I want to go 255/35/20 fronts (which I'd like to fill a bit more the arches), rears can be 285/30 (-1%) or 295/30 (-0.22%). In both cases 10" width rims are good enough to match the 9" fronts.

Note for me:
Dinan went 265/35/20 front and 295/30/20 rear (-1.25%) (on 9.5 and 10.5" wheels).
Alpina went 255/35/20 front and 295/30/20 rear (-0.22%) (on 8.5 and 10" wheels).
Winter standard setup is a square 245/45/18 (which means no issue for the x-drive if 1% difference between F and R).

Cheers and thank you again.