The tire size 225/40R18 can fit a variety of rim widths, from 7.5-9". The rear tire size 255/35R18 can fit from 8.5"-10" wide wheels. Since so many different wheel sizes can fit these tires, the question about what wheel widths (and offsets) you're using is a valid one.
That being said, the 225/40R18-255/35R18 stagger is a very common stagger (perhaps the most common 18" stagger on the market both as OE and aftermarket), used as original equipment sizing on many 3 series cars, so it is not considered to be a big stagger.
Refer to 'Rudy/E46Fanatics' as your previous contact when you order online. Credit E46Fanatics.com by clicking here.