If Mercedes puts the "donut" in a SLK320 as a spare, then there is really no reason we shouldn't be able to use it on our Crossfires. We use the same exact disc brakes, same lug pattern, and the wheels offset is close to ours. The Crossfire also has more than enough inner fender to accommodate a smaller tire.
Yes the SLK320 uses a smaller wheel than we do, but even though their wheels diameter is smaller than the Crossfires, the tires on both cars have approximately the same circumference due to the fact the XF uses a 40 & 35 series tire to the SLKs 55 & 50s respectively.
In Motor Trend, Dec. 2003 issue they did a comparison road test between the SLK320 and the Crossfire. Using the same trannys and axle ratios, both cars were turning 2250 rpm at 60 mph. That can only mean both cars had tires that were the same "height".
TYPHOON55, you were planning on doing the very same thing I was going to do. After reading about "The Crossfire Kids" flat tire experience, I decided I was going to buy the tire repair kit ANDREW recommended, and try to find an SLK spare.