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I destroyed a back tire on my 2008 Subaru Impreza manual transmission with viscous coupling.

The manual says all four tires must be the same size. The spare tire must only be mounted onto the rear axle when applicable. I don't need to pull a fuse to disable AWB on my manual, but automatic transmissions do.

I talked to a tire mechanic who has been doing this for decades. He says that I do NOT have to replace all four tires with new ones. He said replacing just the single blown rear tire with a new one (same general size, but different brand and likely slightly different circumference) will be totally fine. He says he has never had any problems doing it on other Subarus. In fact, the spare tire that I was running on WAS of a different size and it ran on it just fine for 75 miles and no warning lights.

In this case, is it ok to drive around with one new tire on my Subaru? Or do I still have to replace all four?

The mechanic said that as long as the tires are all of the same size (205/55/16) little differences in tread depth and manufacturer differences won't matter.

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  • Welcome to the site. You might want to look into the possibility of buying a part worn. Especially if you can find one that same brand as the tyre on the other side of the car.
    – Steve Matthews
    Nov 19, 2018 at 10:26
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    Even tires of the same size can provide extreme differences in traction, think of a tire made for economy versus a tire made for performance/grip. Spare wheels are only meant for short(er) distances at lower speeds. Always try to have the same tires on the back and/or the front of your vehicle especially on the drive wheels.
    – BossRoss
    Nov 19, 2018 at 11:38
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    Have to agree with @BossRoss - matching brand (and ideally) model across both tires on an axle is highly recommended, especially on more powerful vehicles (you don't mention which model Impreza this is but the wheel size suggests it's likely not to be a WRX or STI). Add in the blithe attitude to mixing new and old and potential for diff wear and I'd be seriously questioning this tire fitter's judgement!
    – motosubatsu
    Nov 19, 2018 at 17:19

4 Answers 4

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Driving around with tires of different rolling radii will cause the viscous coupling to get hot / overheat and eventually fail...

As it will probably happen long enough after the tire fitter did the tire, then he won't have to pay, but you will...

This is a known issue, both on Subaru and Volvo car with awd and viscous couplings, however, you have read the manufacturer's instructions, now it is up to you.

Just out of interest, here is a similar answer: https://mechanics.stackexchange.com/a/51301/10976

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  • Yep, the increased wear on the VC is small but cumulative.
    – motosubatsu
    Nov 19, 2018 at 10:40
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    Note that this also means you should keep all 4 tires at close to the same pressure. One tire running very soft will result in the same effective diameter discrepancy
    – Carl Witthoft
    Nov 19, 2018 at 14:27
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    @CarlWitthoft while you are essentially correct better to say that the tires should be frequently checked for inflation according to the pressures listed in the handbook/sticker on the car. Impreza's do have a front/rear difference in tire pressure to account for the weight distribution and running the rears at the same as the fronts can produce some entertaining handling characteristics!
    – motosubatsu
    Nov 19, 2018 at 16:17
  • Mitsubishi also have the same issue and have different pressures front/rear as well. With AWD it's extra important to read the owner's manual any time you deal with anything tire related.
    – Brian Knoblauch
    Nov 19, 2018 at 17:26
  • It's why I've always replaced my space saver spare with a full-sized spare in all the AWD's I've owned (MPS6/speed6, GC8 TypeRA, r33 GTS-4). Uneven load on viscous awd diffs (and in particular any HALDEX-style system, like in Hondas, VW's etc) will severely reduce their lifespan.
    – Aaron Lavers
    Nov 20, 2018 at 7:01
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It is possible to get new tires shaved down to the same tread depth as the existing worn ones. It may be worth getting that done, then you wont have to replace them all.

For example this article says -

Buying four new tires may be needlessly expensive for drivers who only need a single tire to join the three other moderately worn tires. But you can buy a single new tire from $BIG_TIRE_VENDOR and have the company shave it to the tread depth that matches the depth of your other tires. It will shave any tire you buy from the company, usually for $25 to $30.

Note that shaving a tire will likely nullify its tread-wear warranty. Other retailers may offer a similar service, though the special equipment to shave a tire’s tread isn’t common.

So, no, it is not absolutely necessary to replace all four wheels.

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Subaru's have lots of quirks, and tire size is one of them. Talk to a Subaru mechanic rather than a tire-changer, and you'll get a completely different answer.

Subaru's are well known for having tight tolerances on wheels. If you cannot change all four at the same time, buy a new tire of exactly the same make and model, and have the new tire shaved down to the same size as the others.

The tolerance on Subarus is 1/4" in circumference - which translates to between 1/32 and 2/32 of an inch difference in tread depth (or radius).

You might think that shaving a tire to be smaller is an expensive waste of money. But think of the cost of replacing your center differential (well into four figures - often cheaper to scrap the car), and it's cheap insurance.

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    It's also worth mentioning that circumference is the only measurement that matters. Diameters/tread depth don't correspond directly between different tire models or manufacturers, even before they've worn at all.
    – Shamtam
    Nov 19, 2018 at 18:56
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    Better yet, buy a used tire of exact same model :) Scrap yards sell them for peanuts, and there are "national" databases of available tyres. Quite cheap, does the trick. IMO, shaving is overrated on consumer vehicles, the few mm ov available thread depth vs tire radius is a small fraction.
    – user43417
    Nov 20, 2018 at 4:10
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You don't need to change all four wheels when one gets destroyed. For an emargency run you can use any wheel that fits to the hub. For a regular use I would consider the effects of mismatching tyres on the car reliability and behaviour. You can easily find that saving a buck may lead to a hundred-buck repair elsewhere.

Changing all four wheels for the very same brand, model and dimensions means that all four wheels will behave equally. The ABS wont detect different wheel speeds (no false positives detecting wheelspin/wheelslip) and viscocouples will be loaded only when their opperation os needed.

If you compare tyre radii and they match the couplers and electronics will be fine. The bigger differences in wheel dimenions, the more unnecessary load on the viscocouples and more "thinking" of ABS unit.

Different tyre models and brands behave differently and the harsher conditions the bigger the difference. Try to have same tyres on all four wheels. If not possible (financially acceptable) try to have same pairs on front and rear wheels.

Trust me, you don't want a car to twist during emergency braking or perform any other unexpected move.

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