I've been uneasy at times tinking about the very thin (in my opinion) Subaru AWD tolerance of 1/4 inch in tire circumference from front to back.

Found it difficult to directly measure the tires circumference using a tape. You have to measure exactly at the same place on the tire, and be sure the tape goes straight around the tire, etc.

This morning I went at a college parking lot, measured that 13 parking spaces were equivalent of about 20 tire revolutions. I put a chalk mark on front and rear tires at the point of road contact and proceeded to give them the 20 revs going in a straight line.

If the tires circumferences had a difference of 1/4 inch or more I would find the marks 5 or more inches apart at the end of the experiment.

They were nor more than 1 inch from each other relative to the tire's contact patch.

My mind is now at ease: my viscous coupling is not in danger.

I might try something else next time and measure what effect a difference in pressure does to the rolling radius of a tire. T'was to cold to add that to the experiment this morning.

My contribution to car science :santa:

Please, no Nobel prize necessary.

i think that the vc that is used in the manual has more give to this tolerance than the transfer clutches used in the automatic, hense the lack of a way to disable the AWD when using the spare. i just make sure that i keep them rotated and i wouldn't replace just one...