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I have a 2013 Subaru Impreza with a continuously variable transmission with ~40k miles on it. About two months ago I blew out the front right tire and took it to a tire shop. The folks at the tire shop said the tire wasn't repairable and that I had to replace two tires. The put the new tires on the back and moved the undamaged, but worn tires on the front. I can't remember if the worn tires were 60% worn or 40% worn, but they were the stock tires that I bought the car with.

About a month ago (and maybe 2k miles after replacing the tires) I started hearing a humming/clicking noise from the car (though no indicator lights went off). I took it to a shop and they told me that the noise was coming from the transmission and that I had to take it to the dealership. The dealer told me that the transmission was shot due to the diameter difference between the worn front tires and the new rear tires. Apparently the front tires are 4/32 thread and the rear are 10/32. The dealer claims that the warranty (which covers the powertrain to 60k miles) is void because the tread difference was responsible for the error. According to the dealer any tire shop should have seen the difference in tread and either changed all 4 tires or shaved the new tires down. The dealership wants ~$10k to replace the transmission.

I know nothing about this stuff and have spent several hours reading through conflicting information trying to figure out whether the tread difference could shoot the transmission. Based on some similar questions on this forum it sounds like the differential might be damaged as a result of something like this, but I have yet to see anything definitive about the transmission. Assuming that the tread difference was indeed responsible for the transmission damage (and assuming that the transmission is really totally damaged) is it reasonable to ask the tire shop to help pay the 10k?

  • That's a pretty subjective question. I'm not sure if you can turn it one that isn't.. The site here is more about the technical issues you are having rather than service issues you are experiencing. I'm going to vote to close this unless you want to convert to a technical question. Sorry to hear about your poor experience. Cheers! – DucatiKiller Dec 29 '15 at 3:50
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    @DucatiKiller I would keep the question as it has good a technical explanation. Suby transfer clutches are super picky about tire diameter. One of many reasons I won't own one, but I work on several per day. – Fred Wilson Dec 29 '15 at 5:43
  • Thanks! I didn't know Subaru had terrible AWD design. – doug65536 Dec 29 '15 at 8:55
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    @doug65536 - it's not necessarily a bad design, it's just built to tight tolerances and you have to be aware of its constraints. Properly taken care of, Subaru's AWD system will last a long time. I have a 12 year old Subaru that has never had an AWD problem. The head gasket is another story... – Johnny Dec 29 '15 at 17:16
  • What did you end up doing? all you needed to do was change your worn out 2 tires and get new ones so all the tires are matching. This would have eliminated the sound you were hearing. – Nebelz Cheez Apr 7 '17 at 16:50
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They likely do hold some liability, since they should have known that an AWD should have matching tire diameters across all tires.

They may argue that you hold some responsibility since the Impreza owners manual makes this pretty clear: .

You must install four tires that are of the same size, circumference, construction, manufacturer, brand (tread pattern), degree of wear, speed symbol and load index. Mixing tires of different types, sizes or degrees of wear can result in damage to the vehicle’s power train. Use of different types or sizes of tires can also dangerously reduce controllability and braking performance and can lead to an accident.

However, it seems to be well known in the tire industry that AWD's need matching tires, and that Subaru's tolerances are particularly tight:

http://www.souzastireservice.com/tires-101/tire-matching-awd-4wd.aspx

Subarus have, to our knowledge, the tightest tolerance of any AWD system. Since we see so many, we can tell you what the tolerance is. It can be stated in two ways: The first is 2/32nds tread depth across all four tires. The problem with this is that actual tire dimensions can vary from brand to brand and even from model to model.

...

You may ask what the big deal is? Well, on the Subarus, if all four tires aren't matched within 1/4" around the circumference, you will break the transmission, not maybe or sometimes - it definitely happens.

Here's a tire shop that took responsibility for installing mismatched tires and replaced a customer's differential:

http://www.weareaccurateautomotive.com/blog-0/bid/128677/Tires-for-All-Wheel-drive-vehicle-Beware (in this case it was due to a different tread design)

So to be clear, by replacing the two tires we reduced the circumference differential by .75" and the tread depth by 2/32nds. This is supposed to be good. The speed rating was identical, but the tread design was different.

We concluded that the tread design was the culprit. We replaced the transfer case and installed two more tires to match. Car fixed.

And here's another shop that ended up replacing a transfer case:

http://www.tirebusiness.com/article/19990705/ISSUE/307059999/matching-tires-on-4wds-differences-of-1-4-inche-in-diameter-can

Mr. Albert said he asked the customer: ``Why are you coming back to me? I put on the same sized tire that was on the vehicle.'' The customer replied that the auto dealership told him all four tires have to be the same height and should be the same brand.

Mr. Albert said he turned the invoice over to his insurance company which promptly reimbursed the customer and assured him his tire department had done nothing wrong. But from now on, Mr. Albert said, he's going to insist on installing only four identical tires on four-wheel- and all-wheel-drive vehicles.

So, I'd definitely approach the shop and ask them to pay. If it's a national shop and the local manager won't accept responsibility, look up their corporate contact information and see if the corporate office will help out. Be civil and professional in your communications. If you have a local news media consumer advocate, get in touch with them since public exposure may prompt the company to do the right thing, likewise, you can try contacting them via social media. If all else fails, contact a lawyer about legal remedies.

That said, $10K sounds expensive for a transmission replacement... I would definitely shop around on this (and maybe go with a rebuilt transmission), especially if you end up paying for it yourself. I would have expected only the transfer case to need replacement, though maybe that's built into the transmission.

  • That's a great answer. Cheers! – DucatiKiller Dec 29 '15 at 16:40
  • Wow -- that's a great answer. Much appreciated! – Doov Dec 29 '15 at 18:34
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I had the same problem with my IS250. I would hear a thump during 2-3 shift change. This was because I had 225/50 R and 225/45 F. I changed my tires to summer in April with all matching sizes and the noise went away. Your tranny was not shot. You just had to put similar tire size back.

  • You can't be certain his tranny was ok - there are other vehicles that can have serious damage to the transmission due to tyre differences. You may have recognised the problem sufficiently early to have prevented damage. – Solar Mike Jul 12 '17 at 5:36
  • You're Right! I meant to say to test with regular tires before tranny change. – noble Jul 20 '17 at 18:53

protected by Community Nov 2 '18 at 9:58

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