Originally Posted by DTMenace
The circumference of the spare MUST equal the diameter of the 19 inch rear tires. If this does not match, and it varies in size, it WILL damage the differential. It is not critical when it is used on the front end of the car.
I spoke to a representative of tirerack.com and he explained to me that the reason our xfire's don't come with a spare is because the rear tires are larger than the front ones. Our stability control measures each wheel's RPM and would go haywire if we used the wrong size wheel for the front or rear. Therefore, there's no one size that can ever be used as a spare for all four wheels.