Do some research: I've already had a flat. I hate donuts..
I'm running 18 x 8 front plus tire measures 25.3 in diameter
and 18 x 8.5 rear plus tire measures 25.2 in diameter
I took one of my front wheels off and it fits perfectly into the wheel well. With the size tires that I'm running. So, I'm buying another 18 x 8 so I have a full size spare.
I can now use this spare on the front or the rear, this theory will not work for everyone, but it's worth looking into. Not sure if a tire dealer will sell you a donut, maybe you can find one at a salvage yard or bmw dealer.
Hope this helps.....:thumbup: