Do you own an XC? What does the owner's manual say?

Almost all, if not all AWD (and any other vehicle that is more than one wheel drive) have specs about what differences in tire diameter or circumference that they will tolerate without too much damage to the drivetrain.

The exact process for each drive system varies, but the results are the same - do not use different circumference tires on a AWD (4WD) drive train.

Subaru says 1/4 of an inch.

While differentials accommodate wheels turning at different speeds, there is a cost.

The cost is mechanical wear and heat. You wear out parts in the drivetrain and the heat damages the fluids and parts. You then have early failure.

Actually, newer vehicles are much better than older AWD drivetrains. Most older AWD (4WD) trucks had transfer boxes and hubs to disconnect as many parts as possible because of wear and heat just driving around.

Tire Rack has a simple explanation (basically the same as mine) and what you can do if you just need one tire. A new tire can be shaved to match your other tires.

If I had a flat on the drive axle of my vehicle, I would move a full sized tire to the drive axle and put the space saver on a non drive wheel position.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa