255/35/18 on an 18×8.5″ (ET50) rim is the stock rear size on my E46, though probably not on any E46 xi models. The stock front is 18×8″ ET47.
I have run both 255/35/18 (17×8.5 ET50) and 245/40/17 (17×8.5″ ET40 rim) tires on the front and back of my E46. Sometimes in front it rubs the inner fender (towards engine bay) at full steering lock so I put in a 5mm spacer and have been waiting for some 12mm spacers. On the backs if you use ET40 that’s sitting 10mm farther outward than stock (ET50) so on a lowered car it can run the fender liner or fender lip. With a non-lowered xi, I doubt that would ever be an issue.