So my new wheels and tires for summer come on Friday. As luck would have it, I put a non repairable gash in my already worn out rear pirelli (315/35/20) today. Not replacing it as my my new setup is only days away. How bad would it be to use one of my original 18" wheel and tire (255 size) as a spare for the next hundred miles or so on the rear until the new ones arrive? Would I be damaging anything?
Thanks in advance!

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01 X5 3.0i (gone)
04 X5 4.8is (gone)
06 X5 4.4i (gone)
09 X5 35d (current)
12 X5 35d (current)