I just replaced a damaged tire on my 2000 subaru forester. The person at the tire shop told me that I should always replace my tires as a set, since any varation in tire wear greater the 3/32 could cause tramsmission damage due to the differance in rotating tire speed with the all wheel drive. Is this correct.
Yes.
Yup!
What he told you is definitely correct.
If you don’t believe the tire guy or me, you could try reading the Owner’s Manual (perhaps for the first time?), which will verify this advice.
If you cannot afford to buy 4 tires, you can have the new tire “shaved” to match the amount of tread wear on the 3 older tires. Not every tire shop has this type of equipment, so you should call the biggest tire shops in your area to determine who can provide this service for you. However, don’t delay in doing this.
If you drive more than 50-75 miles on those mis-matched tires, you will very likely wind up with a repair bill of…around $700.
This is one of the “advantages” of AWD car salesmen don’t talk about…
What does your owner’s manual say?
Your owner’s manual should be your guide on this.
And, if after measuring the differences in diameter you’re still unsure, be advised that three new tires (to match the new one) are far cheaper than a new transfer case.
If you are hard up for funds you could look at a used tire.
…so no one asked how worn out your other tires are? At 10 years of age the car could easily have gone through many sets at this point.
If the tires are identical make/model/size (which hopefully they are!!) then you can measure the tread depth… if your existing tires measure “X” and your new tire measures “X+Y” then you know that “Y” is the difference in height that your drive train will experience.
Do check your manual though–3/32 sounds like too small of a difference to cause a major problem. I’ve put 5000 miles on a 2005 Subaru Outback Sport with one snow tire on the front and three matched all seasons on the other wheels… it was meant as a temporary solution, but apparently I blacked out for the entire summer when it came to that tire… no ill effects so far.
From: https://www.subaru.com/my-subaru/faq-technical.html
"4. Do I have to replace all four tires on my AWD Subaru?
All of the tires on your AWD Subaru must be within 1/4 of an inch of rolling circumference (part that touches the road). This is because of our All Wheel Drive System."
So, to put things into common denominators… Subaru says you’re OK with a difference up to 8/32" … your tire sales person says 3/32" … My guess is that that Subaru 8/32" value already incorporates some safety margin, so more than likely we’re looking at a case of the 3/32" value being there in a best case as a “cover your ass” for the tire place to keep out of damaged differential claims, and most likely case: to sell more tires.
So it really depends on the condition of your existing tires.
BTW, I in no way condone driving 5000 miles one different tire as I did 
One of the most expensive things you can do to a Subaru is cheap out on replacing tires.
You are comparing apples to oranges.
Subaru states 1/4" ROLLING CIRCUMFERENCE and tire shop states 3/32" of tread wear which mean DEPTH. That are likely roughly equal.
One way out of this would be to have the new tire ‘shaved’ to the identical size as the others, google ‘tire shaving yourcity’, see if there are any shops near you that do this.
Gee–I couldn’t have said it better myself.
Oh, that’s right–I did, at 7:10 PM!
;-))
Oooooops - short attention span here…