My spare tire is the same size (185 SR14) as my four mounted tires, but it
has less remaining tread depth.   It is also a different brand, Michelin XZX
instead of Bridgestone.  I have measured the circumferences and the spare is
2% smaller than the mounted tires.

Am I taking a big risk of viscous coupling damage if I need to use the
spare?  Would it make a difference if the spare is run for a maximum of 50
miles before the mounted tire can be repaired/replaced?

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Sky-Blue Waters)