You can have slightly larger tires on the rear than the front. Smaller rear tires will confuse the ABS into thinking the rears are slipping (rotating faster) and try to compensate.

The bevel gear (transfer case) adds and removes power depending on wheel speed for each wheel. If the rear wheel(s) are spinning, more energy is routed to the rear wheels.

Different tire sizes, like putting a spare on the right rear, will cause the bevel gear to implode after 50 miles. It will over heat the fluid and start cooking parts and seals.

My 1998 V70 had AWD and never gave me problems up to 188K miles.
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