8 - 7.5 = .5 inches wider
.5 / 2 = .25 inch offset adjustment.
.25 x 25.4 = 6.35 mm offset reduction
therefore you need 36 mm offset.
Tire rubbing will depend on the size of tire you install. at the recommended 215/40 R17 you shouldn't have rubbing, but with a 8" rim width, the tire will look wider at the rim than the tread. At a 225mm width, you will need to file down the splashshield retention bolts to allow sufficient travel.
thanks a lot, but i was planning on going 235/40/17 instead of the normal 215/45/17. I have read info on saabnet.com and people have done 235/40/17 with no rubbing. I researched a little on tirerack and the heights are the same, and the 235's are obviously wider. Would it be rubbing because its too tall or too wide....or both? thanks, will