I think our car has the big wheels more for looks than caliper clearance. Therefore as a spare you wouldn't need such a big wheel. I haven't looked at the calipers yet, but I'd imagine that a 17" wheel would clear the brake calipers on our cars. Isn't 17" what the SLK320 has anyway? And I assume the xfire has the same brakes, right?

So a 17 would probably fit, maybe even squeeze on a 16.