different size tire all the way around on AWD can burn up the transfer case and/or the front or rear diff. front wheel drive with a mini spare can burn up a transmission which is why you are supposed to mount the spare on a non drive wheel. rear wheel drive with two different sizes side to side can burn up a limited slip, or a transfer case if the front tires are smaller/bigger and you put it 4WD. it has to do with the RPM of the tires (revolutions per mile) and some cars are very sensitive to mismatched tire sizes.