Now I will stand corrected on this but you should watch that. If running diameter is different then your circumference well be different and you could load up your limited slip diff. When dealing with circumferences you are dealing with the factor of PI
Say a tyre had, for example only, an outside diameter of 600mm. The circumference would be PI x D = 1885.2mm
Now lets say you had a tyre with an outside diameter of 605mm. The circumference would be PI x D = 1900.9mm
There fore the smaller diameter is going to want to spin faster than the larger diameter for the same distance travelled.
Maybe not a big issue as a spare, but if you had to drive some distance..............
Just a thought.