On the inside of the doorsill in the xfire it says to use no smaller than 18 inch wheels, as I recollect, but I don't know why.

I would like to have smaller wheels too, the same size in front and back. It's more practical, considering the cost and limited availability of replacement tires, especially in the event of a flat ; the new SLK comes with 17s I believe, and I don't see why the xfire at 215 hp and 3100 lbs needs 19" wheels, but I don't understand all the parameters.