[gangstead]

Unlucky me had a blow out on one of my tires that only have about 6k miles (out of 90k life) and the mechanic gave me the standard "it's AWD so you need to change all 4" disclaimer. After he measured he said that they were 12% gone but I didn't think to ask what that meant in terms of 32nds tread depth. I'm not sure how % is calculated... is 0% equal 0/32 tread depth or is 0% the state minimum 3/32"?

Doing my forum research there is a claim that Subaru says you are OK to replace just one tire as long as circumference is within 1/4" which translates to ~2/32" tread depth. I went to my owner's manual (2010 OB CVT) for confirmation but it says that they must match, with no tolerance specified (similar to this language on Subaru's website).

What I'm wondering is did Subaru drop the 1/4" circumference tolerance at some point? Perhaps due to design changes or overzealous lawyers? Or was this never an official policy?

My forum research can be summarized in two groups: those who had a menagerie of mismatched tires 300k miles with no problems against those who had one wheel drive through some latex paint causing slight diameter increase resulting in torque bind and transmission melt down within a mile.

 

[idosubaru]

My forum research can be summarized in two groups: those who had a menagerie of mismatched tires 300k miles with no problems against those who had one wheel drive through some latex paint causing slight diameter increase resulting in torque bind and transmission melt down within a mile.

HA HA! latex paint...well done! ignore both of those groups.

Subaru's stance and the 1/4" circumference mark stands. Your owners manual isn't a rewriting of that probably so much as just ambiguity and simplicity of a one-size-fits-all recommendation.

Truthfully, based on the way the differentials physically operate, replacing one tire won't matter. But few people can tolerate that because they don't understand differentials. Ignorance is bliss.

Options I would suggest:

1. Measure the tread depth first and get an accurate reading here, if it's in spec's, just get a new tire.

2. Buy a used tire that's close to the same tread depth. They are wildly available on ebay, inexpensive, and ship to your door. Or call around locally, Outback used tire sizes can be hard to come by in some areas.

3. Get two new tires and put them both up front - they will shortly wear down to within the relatively lightly worn rears.

This isn't a big deal. Most of the commentary you read is arm chair regurgitation and nearly political "The constitution of Subaru says". That's fine and that works, but if one wants to learn other options they are available and offer more long term value to those who have the capacity to learn, understand, and look beyond Subaru for maintenance.

Subaru does not govern the laws of physics. I certainly start there and favor them highly in most regards, but there are plenty of cases where there are more options, better fits, or more reliable repairs, or more options, than what Subaru calls for. In general - Subaru is the best "one-size-fits-all" resource by far and will always be quoted as such.

 

[plain OM]

No, I don't think Subaru has changed or dropped the requirement -- the 1/4-inch figure has never been included in the Owners Manual as far as I know.

Most AWD car makers have similar recommendations; the actual figure might be slightly different (some use circumference, others diameter or radius), but the underlying reason is the same.

With only 6 K on the original tires, there's a chance that the difference in tread depth on the originals and on a new replacement would be less than the spec but the only way to tell is to measure. Ideally measuring the circumference, but tread depth can be used as an initial substitute.

I would imagine "12% gone" means if the original tread was 10/32 deep, the treads on tires are now at around 8.8/32 deep. That's right at the margin -- a 1.3/32-inch difference means a 1/4-inch difference in circumference.

(The minimum requirement, when the wear bars show up at the surface, is not part of this calculation. In other words, even though the tire tread, when new, might be 10/32-inch deep, the tire is safely usable only until the tread is down to about 2/32-inch depth, which is the height of the wear bar.)

When measuring tread depth with a tire tread depth gauge, it's important to avoid setting the gauge on the wear bars at the bottom of the tread as this will give a lower tread depth reading.

As far as damage is concerned, I don't recall any reports of mechanical failures in a short time due to slightly different circumference tires. (The latex paint story is not one I have seen but it's more likely there was damage before.) Slightly different size tires subject the AWD system to additional wear. The effect won't be sudden; rather it will be cumulative, with the damage becoming apparent only well down the road. That's why the temporary donut spare tire can be used for short distances -- the impact will be minimal if the use is limited.

Not sure where you are with this now; have you replaced the tires, or is this still pending? If still pending, then perhaps redo the tread depth measurements yourself. Tread depth gauges are sold at most stores carrying auto parts and accessories, and cost only a few dollars. Check each tire at several places. Then go to the tire manufacturer's website to find the specifications for that tire -- it will usually indicate the original tread depth.

Let us know what you find and/or do . . .

 

[Unclviny]

My local 4WD tire shop has saved me a few times, measure the 3 remaining tires and shave the new one to match.
I would MUCH rather leave some good rubber on the tire shop floor than buy 4 tires or pay to have the drivetrain fixed.

Vince

 

[SE95]

My local 4WD tire shop has saved me a few times, measure the 3 remaining tires and shave the new one to match.
I would MUCH rather leave some good rubber on the tire shop floor than buy 4 tires or pay to have the drivetrain fixed.

Vince

To me, that is pointless. You PAID for the tire, then you PAID to have someone remove some of the tread. You might as well take some money out of your wallet and light it on fire. Unfortunately it is the nature of the beast to own one of these cars... you can't mix tires, four identical tires must be on the vehicle at all times. Believe me, I'm a tire salesman and I get told that I'm full of **** on a daily basis because I tell customers with an AWD vehicle that we can't put just one or two tires on. Subaru owners are better than most and for the most part they know before I even start talking that they need four. But sometimes I wonder if car salesmen make that clear when the customer purchases the car.

At least if you buy four tires,you take of three tires that are still in good shape, and you can get a few bucks out of them yet throwing them up for sale on Craigslist or the like.

 

[Unclviny]

OD is OD, and I pay out A LOT less this way (I always run matching tires!).

Vince

 

[idosubaru]

To me, that is pointless. You PAID for the tire, then you PAID to have someone remove some of the tread. You might as well take some money out of your wallet and light it on fire.

shave one tire or get rid of three - either is a waste of money. not everyone sells on craigslists, and one would have to do a cost analysis to determine if there's any benefit. it would surely depend on each circumstance, it won't always be the same.

you can't mix tires, four identical tires must be on the vehicle at all times.

yes you can.

why does tread pattern need to match?

at 60 mph driving straight on the interstate - wheels of the same diameter are turning at the same RPM's. changing the tread pattern does not make a tire turn at different rpm's while driving down the interstate, it doesn't add up.

all that's needed is a circumference match.

 

[SE95]

shave one tire or get rid of three - either is a waste of money. not everyone sells on craigslists, and one would have to do a cost analysis to determine if there's any benefit. it would surely depend on each circumstance, it won't always be the same.

yes you can.

why does tread pattern need to match?

at 60 mph driving straight on the interstate - wheels of the same diameter are turning at the same RPM's. changing the tread pattern does not make a tire turn at different rpm's while driving down the interstate, it doesn't add up.

all that's needed is a circumference match.

They need to match because the vehicle manufacturer says they need to, the tire manufacturer says they need to, the Tire Industry Association says they need to, and a big army of lawyers also says they need to.

It is a HUGE liability for any shop to mix tires on an AWD or 4WD vehicle. If a shop mixes tires even though they know better, and even though you said "I don't care I promise if it blows up I won't sue", it means exactly jack. When the car does **** up, or you lose control of the vehicle when something goes completely haywire in the system, the differential locks up, trans locks up, and you run a minivan carrying 5 kids off the road when you lose control and they're dead... the family, insurance company, and lawyers of both aren't going to care that "it probably wont cause problems". And they won't be suing you, they will be suing whoever put the tires on that should have known better.

If you want to buy your own equipment, your own tire machine, your own balancer, then go for it, mount your own tires. But it isn't a risk I'd take.

 

[idosubaru]

They need to match because the vehicle manufacturer says they need to, the tire manufacturer says they need to, the Tire Industry Association says they need to, and a big army of lawyers also says they need to.

It is a HUGE liability for any shop to mix tires on an AWD or 4WD vehicle. If a shop mixes tires even.....

Copy that, that's a good reply, well done. If I owned a shop I would not at all care to be at the expense of an uninformed, ill experienced, and selfish customer. There are plenty of those. If I owned a shop I would take the same stance you do.

If you want to buy your own equipment, your own tire machine, your own balancer, then go for it, mount your own tires. But it isn't a risk I'd take.

And that is what I do. I either install and balance my own or have a shop do it....but it should be well understood and implemented by someone who knows what they're doing on one particular car and will continue to maintain that car properly.

 

[mrk610]

I feel your pain. Bought new tires in Sept at local sears. I was supposed to get the GY TTT all season. I got the old GY TTT . Did not know untill I got home and called sears . They refunded me the difference in price plus 10% for there mistake. So I saved about 36 dollars per tire. I also asked them , what about replacements if I have a bad tire. They said no problem. 4 k mile later I get a slash in the pass front tire. I call sears and guess what the new sales manager said . The previous one that sold me my tires got fired for making false statments to customers.
GY stopped making those tires 15 months ago.
So I had to replace all 4 tires again. Luckily I got a great deal from a GY tire center about 45 mins away from me.
I now have the GY TT all season on my car and I love them . What a difference from the old tt to the new tt.

Mike K

 

[gangstead]

Not sure where you are with this now; have you replaced the tires, or is this still pending? If still pending, then perhaps redo the tread depth measurements yourself. Tread depth gauges are sold at most stores carrying auto parts and accessories, and cost only a few dollars. Check each tire at several places. Then go to the tire manufacturer's website to find the specifications for that tire -- it will usually indicate the original tread depth.

Let us know what you find and/or do . . .

The tires were defender XT's which my Costco no longer has in stock. They ordered one replacement and are covering it under warranty. I'm just paying the $22 installation fee. If I decide to go for 3 more that's on me. I have until Wednesday to decide. My plan is to do the tread depth measurements myself.

 

[plain OM]

Looks as if the Michelin Defender tread depth is 10/32. I presume this applies to the XT which, it appears, is Costco's version.(Defender | Michelin Tires) So if your current tires have, say, 9/32-inch of tread depth (measure at several places around each tire) then you're probably good-to-go.

If you want to have even greater confidence, measure the circumference of the three current tires and the new one after it's mounted and inflated.

See the attached for method. (All four tires should be set to the same psi for this.)

 

[tdelker]

Use a tailor's tape measure (you know, the cloth kind) and just measure the circumference directly. That's what matters.

Tom

 

[rharvey]

If I understand correctly, measuring the tread depth assumes the outside of the carcass of the replacement tire, regardless of brand, is the same as the tire you are replacing...is this a manufacturing spec??? Thanks for your help...rharvey

 

[1 Lucky Texan]

what matters is 'rolling circumference'(or, 'rolling radius'). other, easier, measuring methods may be 'close enough' but ,the more remote/different from actually measuring rolling distances could lead to false confidence. because of differences in sidewall stiffness and rubber compounds - trying to make a different brand/model tire work with just a circumference measurement might cause problems.

the most frustrating part is, no one can really say X percent difference used for Y miles will do Z amount of damage to the trans.

with options like used tires, tire shaving, putting odd-sizes on opposite corners, etc. There really should be no reason to mix in a single different brand/model tire for extended use. If you MUST, then find some long road section or parking lot, mark the bottom of each tire, drive straight forward x revolutions of a target tire, compare marks around the car, maybe try small air pressure adjustments until all tires roll the same. (if I'm thinking right, after 40 revolutions, if every mark is within 10" of the ground, along the arc of the tire, that should be within the 1/4" figure. right????)


certainly driving with 'drastically' different tire pressures leads to its own issues.

 

[arthuruscg]

5 tire rotations FTW!

 

[rharvey]

Yes, I can appreciate that circumferential agreement is essential....to find a replacement for half a partially worn tires would require having a replacement mounted and inflated in order to get an accurate measurement...this might require some trial and error mounting...my concern, since we make long trips, if we needed a replacement on the road somewhere, for a more expedient qualifying method....IF all tire carcass diameters at the base of the tread were the same, then tread depth would be a close and convenient guide...if there is no manufacturers specification that precludes that diameter measurement then tread depth measurement would not be worth considering, it could vary considerably and then seriously alter using it as a guide for final, mounted, pressurized circumference

 

[1 Lucky Texan]

getting a 5th matched, fullsize spare and keeping it in the rotation with the other 4 so treadwear matches, might be the best suggestion for you. It may be tricky to fit/keep in the car. Some people have managed it when the tire is deflated - but they carry an emergency inflator too.

more info here; http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18&

 

[arthuruscg]

We keep a small electric pump to air the spare tire back up.

 

[jogosub]

As far as I know Subaru doesn't offer any warranty on tires that come with the car. Tire manufacturers do.
You will find various leaflets in your glove box, when you buy a new car, from various tire manufacturers as Subarus come with different tires depending on region, where the car is sold. The warranty or pro-rated warranty is right there on that brand's leaflet. There is no "uniform" rule on replacements....