[David70]

I never bought into the AWD needing four exactly worn tires either. The number of people that don't rotate their tires, buy mismatched brands at different times, etc. would have the huge numbers of AWD cars all with problems. Then, put on a space saver spare where there is almost no chance of it being within 32nds of matching the rest of the tires and it would also destroy diffs quickly. My guess is some of the space savers are off by a huge amount, many times greater than the new to worn tire question.

While I don't think it will hurt you diff I would replace it. Normal tire comes with about 12/32" of tread, if you are saying the worn tire has 25% left, 4/32", it is worn enough to not deal with.