Thanks for the information it's very interesting. My question was prompted when I discovered that the 4.6is and 4.8is editions came with temporary spares and not full size spares. Further, those temporary spares are 19" spares. Given that a temporary spare takes up less space in the trunk than a full size 17" tire, I thought maybe I would swap my full size 17 for a temp 19" which coincidently would go well (I think) with my 19" sport wheels.

I'd be very interested in finding out BMW's reasoning for putting a 19" temp spare in the 'is models which as everyone knows have 20" wheels. Maybe they felt that the 17" full size was too big a difference from the 20"? And maybe an 18" or 19" full size spare was either too big for the trunk or too expensive?