Quick question as I sit here at the tire shop...
 

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!

No harm; think about it, it's ok to use oem compact spare which is what, 155 in width?

Look up either" "revs per mile" or "outer diameter" of EACH tire on tirerack.com...this will tell you how much abuse the different rotations will cause with your differential.