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I have a 2014 Forester Premium. I had replaced 4 Goodyear-Viva-3-All-Season-Tire-225-60R17 tires about a year and half back at Walmart. I got a flat (ran on a nail) on one of my tires. Walmart could not fix it, so they put a new tire, free of charge though as I had some road hazard insurance. I have put about 13k miles since I had put them.

I told the mechanic that mine was an AWD. He said it was not a problem. I had him check the tread on my tires. The 3 old ones had a 7/32 tread and the new one had a 8/32 tread. He said may be the new ones start out at 8/32.

Now that one of the tires is new, should I be concerned about damaging my transmission?
 

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2016 XT Premium
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New ones probably start out at 10-12/32 and if you are so inclined you can call TireRack and they can shave down a single tire to 8/32 and send it to you. I wouldnt run a difference of more than 3/32 long term to protect the differentials
 

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2019 Forester Sport Lineartronic® CVT
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Your tires can be perfectly matched and things like turns will alter how far each goes in relation to the other 3 every time. Only if tires were solid like rail road wheels of steel running on steel rails in a straight line from start to finish would they all 4 see the same mileage. When you just make a left turn out of a parking lot or in a mile long left curve going out the interstate, the outside tire travels further than the inside tire & the front axle travels further than the rear axle. My cars seldom go far on perfectly straight roads. Tires are loaded slightly different, air pressures vary.

Just put a new tire of same size on it and go.
 

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2018 Forester
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The only thing that 1/32" will affect is the spider gears in that differential will turn very slightly while going straight ahead. Now if you put 2 new tires on one end and left 2 old ones on the other end then that would have an effect on the viscous coupling.
 

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2004 FXT 4EAT
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563 Posts
From my understanding, when Subaru introduced the space-saver spare tires, they also made some modifications to the differentials to ensure you wouldn't damage them if you ever needed to put the spare on. So, I wouldn't worry about it.

I would however put the new tire on the back, so your front wheels are the same diameter.
 

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2016 Crosstrek Premium
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What I would suggest,to avoid damage to drive train and to keep tire wear uniform is this.Put the new tire on the front,replace the other front tire and use that one for a full size spare.Doing this will be exactly the same as if you had just rotated the tires.The new tires on the front will soon wear down to match the rears.Running an oddball tire will eventually damage the diffs,note the speed and range limits on the OEM spare,and will likely effect how the car behaves on slick roads.
 

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Discussion Starter #8
The only thing that 1/32" will affect is the spider gears in that differential will turn very slightly while going straight ahead. Now if you put 2 new tires on one end and left 2 old ones on the other end then that would have an effect on the viscous coupling.
When you say 'one end', you mean front or rear?
 

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2016 Foz CVT
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Your tires can be perfectly matched and things like turns will alter how far each goes in relation to the other 3 every time. Only if tires were solid like rail road wheels of steel running on steel rails in a straight line from start to finish would they all 4 see the same mileage. When you just make a left turn out of a parking lot or in a mile long left curve going out the interstate, the outside tire travels further than the inside tire & the front axle travels further than the rear axle. My cars seldom go far on perfectly straight roads. Tires are loaded slightly different, air pressures vary.

Just put a new tire of same size on it and go.
This is, in general terms, bad advice.

Most drivers spend miles on the highway and that's when differences in tire circumference does damage. Low speed turns or long sweepers don't matter. Framing it in terms of 'needing to be on rails' or perfectly straight is also incorrect. Even rails are never perfectly straight. There is a threshold of sorts where inside it you're safe, and outside it you can do damage either over time or quickly.

Subaru is pretty clear about their published tolerances and requiring a tire to be exact same brand/model.


From my understanding, when Subaru introduced the space-saver spare tires, they also made some modifications to the differentials to ensure you wouldn't damage them if you ever needed to put the spare on. So, I wouldn't worry about it.
The spare is intended for low(ish) speed short term use. Just enough to get you to a shop. Mounting an odd tire is long-term and therefore much increased potential for damage depending on the circumferential difference.

Subaru has also used space-saver tires since at least the early 90s, BTW.

Now, more applicable to the situation here, if a new tire of the same brand and model is only 1/32 difference, then you're fine. The specs have historically been a tolerance of 2/32 is allowable. Same brand/model tire is a hard rule. The tread compound must be the same otherwise they can wear very differently.
 

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2006 Forester 5-speed manual
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There's only one answer here...

Don't worry about it! Not an issue! I've run a completely different tyre size for some months with no issues.
 

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I have a 2014 Forester Premium. I had replaced 4 Goodyear-Viva-3-All-Season-Tire-225-60R17 tires about a year and half back at Walmart. I got a flat (ran on a nail) on one of my tires. Walmart could not fix it, so they put a new tire, free of charge though as I had some road hazard insurance. I have put about 13k miles since I had put them.

I told the mechanic that mine was an AWD. He said it was not a problem. I had him check the tread on my tires. The 3 old ones had a 7/32 tread and the new one had a 8/32 tread. He said may be the new ones start out at 8/32.

Now that one of the tires is new, should I be concerned about damaging my transmission?
I was in exactly the situation you are about a year ago. My flat was in the front. I put the space saving spare on the front to continue home (20 miles). I thought the trans-differential were going to come apart. I quickly move the spare to the rear. I replaced the blown tire with a brand new one of exactly the same brand/type with a little more than 1/32 difference on the rear. I have put on about 10,000 miles and have noticed no difference at all; no noises, clanks, lags, etc. I have a 2006 Baja Turbo.
 

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Subaru says if you get a flat you must put the space saver on the rear, so if you got a flat on the front you'd have to move one of the rear tires to the front, then put the space saver on the rear. I absolutely HATE space saver tires, and am going to save the best of the OE tires on my wife's '17 for a full size spare.
BTW, I did this 20 years ago and still have the spare for her '98 Legacy Outback, on a Subaru alloy rim, in like new condition, anyone wanna buy it lol?
528918
 

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2011 Forester 2.5X Automatic
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Well, here's what a little bit of online sleuthing has brought about: no more than a 2/32 difference in tread depth across all four tires or no more than a 1/4" difference in circumference between the tires.


Do an online search with "subaru awd tire diameter difference" and you'll find some multiple sources with these specs. Go to a Subie dealership site and they will be somewhat nebulus in their declarations about having to buy four new tires versus just buying a single tire.
 

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2015 Forester Premium CVT
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I have been in your position. The mechanic put a new tire and claimed all is fine. I ran like that for a few months but it kept making me uncomfortable that something could go wrong, sooner or later, due to minor size variation. I found a guy who would shave the tire and after $40 and an hour back and forth, all tires have the same tread depth.

IMHO, you are going to find all kinds of responses here. Your vehicle, your wallet, and your responsibility. Follow the book and try to keep all treads similar.
 

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I will not add conflicting advice since I think all angles have been covered.

I will suggest that the original poster and everyone with more than 20,000 miles or so, measure the depth of the tread ourself. What we do with that information depends on which of the above camps we agree with.

I suspect most are off more then we would guess. I also would not depend on a Walmart technician.
 

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Sahuarita, Arizona USA 2018 Forester Limited
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I recently noticed a nail in a tire the day before an appointment for a 30k mile service at my Subaru dealer. The nail was near the edge of the tread so I was prepared to hear that it could not be repaired. My dealer can obtain shaved tires so I told my service advisor to give me a shaved tire quote if the nailed tire could not be repaired. When my service advisor came to the waiting room to confirm that the tire could not be repaired, he stated that there was no need to shave the new tire since it would be within 3/32 of the other three tires. He indicated that 3/32 was the maximum difference allowed. I took his word for it because I have a good rapport with this service advisor who has been at his job for many years.
 
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