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I have a 2005 Subaru Legacy 2.5i Wagon.

I just destroyed the passenger-side rear tire.

The tires have approx. 6/32" tread remaining, per an informal measurement with a coin (I used this guideline from tirerack.com)

These are Nokian tires, and according to the specs I found online, they have 11/32" tread depth when new.

Another article on tirerack.com on this topic, suggests that Subuaru states that the differential among tread depth should vary no more than 2/32"

My owner's manual does not specify the what difference in tread depth tolerance is, however they do point out that I have a dashboard indicator "AWD" that will flash if the vehicle is driven with tires of different diameters, and I assume, the drive train is in danger of being damaged.

Among my family and friends, I've received conflicting opinions regarding whether I should replace all 4 tires immediately, or if I could potentially purchase the same brand and model of tire, whose tread depth, I assume, would be 11/32" and be different by approx. 5/32" from the existing tires.

Does anyone have a definitive answer as to which option I should choose?

Is it true that tread-depth can influence tire circumference enough that driving my Subaru for an extended period of time would damage the drive train?


CONCLUSION: I chose to replace all 4 tires with new tires, and I'm happy I did.

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I do not have a definitive answer, so will leave this as a comment. While it may seem wasteful, you could always just buy one new tire and have it shaved down close to what the other three tires are. This would be a lot less expensive than having to buy four new tires. I would not suggest running different size tires on your AWD Subie ... but that is my opinion and not a definitive answer. –  Paulster2 Mar 23 at 23:46
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I've just read a few posts about this topic, but in the end I believe the best thing for you to do is to phone the Subaru agent and ask them what the difference in tread depth can be. Don't make the mistake of asking if you should change all your tires because that gives them a gap to sell you a new set. What you want from them is information. –  Juann Strauss Mar 24 at 9:31
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@jmsmcfrlnd ... I think shaving is common enough. A lot of people who do weekend track racing (closed track road course) do it to their brand new tires. On the track it provides better traction, though it definitely reduces overall tire life. –  Paulster2 Mar 24 at 9:51
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I had another thought ... you could also see if there are any used tire shops in your area which might have the tire you're looking for ... for a reasonable price, you could be back rolling again. –  Paulster2 Mar 24 at 21:26
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@Paulster2 I did end up at a used tire place, but they did not have a sufficient match that suited my comfort level. I also discovered I could have purchased 4 used tires as well, as a means to get by. –  jmsmcfrlnd Apr 8 at 23:34

1 Answer 1

up vote 4 down vote accepted

Does anyone have a definitive answer as to which option I should choose?

I do: you need matching tires.

That said, it's your car and you have to make the decision. I'd tend to replace all four. @Paulster2 also makes a good point that you could shave down a new one to match.

Let's start by looking at one of the most useful paragraphs from the Tire Rack article that you cite:

Four-wheel drive and all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail.

What that means is that a mismatched tire won't break your drivetrain immediately. It's more subtle than that: at least one differential will be loaded all of the time and at all operating speeds. So, instead of managing the different rotation speeds in a relatively low speed turn, you'll be cooking your differential at highway straight line speeds.

My owner's manual does not specify the what difference in tread depth tolerance is, however they do point out that I have a dashboard indicator "AWD" that will flash if the vehicle is driven with tires of different diameters, and I assume, the drive train is in danger of being damaged.

It's good that there's a light but, frankly, you can hear the differential being overworked if you listen for it. On my Subaru, if I inflate the rear tires to match the front tires' pressure (approximately 4 psi up), I'll hear a distinctive high-pitched whine from the center differential every time I coast down from speed in second gear.

Note: my car is a manual transmission WRX. You may have different audible symptoms in other vehicles and transmissons.

In the end, the reason that the tires should match is the same reason that you shouldn't tow an AWD car (instead they should be flat-bedded). If the back wheels are turning at speed for miles but the front ones are not, the center differential will burn itself up in short order.

Is one mismatched tire that extreme? No, of course not. It will wear your differential, make worrying sounds and affect your handling.

You still have to make the call, though.

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Great last line of your answer; and I did just that: I made the call. I chose to purchase 4 new tires. I don't like worrying of any kind. :) –  jmsmcfrlnd Apr 8 at 23:33
    
@jmsmcfrlnd, I'm glad to hear it. Hopefully this set will live their entire natural life! –  Bob Cross Apr 9 at 0:32

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