Buying an 18 inch BBS lm will look like 17's ...remember the step lip step lip setup will create an illusion of a smaller wheel because of the smaller center...jmho
++1
if you're thinking of going 18" BBS LM, I would rather keep the 18" stock.
To my knowledge there are no reps for RC. 18" RC looks bigger than 18" BBS LM because it spoke goes all the way to the outer edge.
also for the offset it would be depend on the wheel width, not the diameter.
If you look around the picture other member posted you can get a pretty good idea of certain offset looks like with certain wheel width.
But just go give general overview of the most common offset I've seen
M5 OEM
18x8.0 ET20
18x9.5 ET22
BBS RC (Discontinued)
18x8.5 ET15
18x10. ET25
BBS CH
19x8.5 ET20
19x10. ET20
BBS RS-GT
19x8.5 ET15
19x9.5 ET25
19x10. ET25
(M5 with 19x9.5 all around)
BBS LM
19x8.5 ET17
19x10. ET25
BBS RGR
19x9.0 ET20
19x10. ET25
AC Schnitzer Type III 2 pc
19x8.5 E15
19x9.5 ET21.5
CarGraphic
19x9.0 ET20
19x10. ET20
Hope this give some idea what partcular offset and wheel width looks like on E39 M5
Regardz,
J Irwan