You should still rotate your tires regularly. That's not what they're saying. Follow the instructions in your user manual.

You shouldn't mix and match different size tires, even to the extreme of not just replacing one or two when you get a blowout.

It would destroy your AWD system. You'd end up needing a new transmission. There are lots of good technical explanations about how and why on here and every other Subaru forum and also on YouTube if you're interested in the details.

 

If your rotating tires/wheels every oil service, don't sweat it. Keep the proper pressure in them and they will last a long time. It takes us forever to go through tires.

 

You're not going to fly off the road but you might end up with an expensive repair bill. Nearly all Subaru's, and other full time all wheel drive vehicles from the last decade, read the wheel speed of each wheel and make many decisions behind the scenes based on those readings. If your tires are different diameters the vehicle can be mislead into making some poor choices that can stress the drive train. You can and should rotate your tires per Subaru's guidelines. But you should not replace just 2 tires when the other 2 are significantly worn from not rotating them correctly. Likewise if one tire is damaged you should have the replacement shaved down to match the other 3 tires.

 

Hoping for some help here. Recently had a flat in El Paso, we live 3 hrs away, very remote. The sidewall was compromised so we had to buy a new tire. Bought a brand new, different brand, (not the Geolandars that came on our 2021 Sport), but the same size as the other three. Our Crosstrek has almost exactly 30K on the clock, 95% highway miles. My concern is tread depth. Though the new tire is the exact same size as the others, will tread depth difference have a similar effect on the transmission like a different sized tire/wheel would? Really hoping I don't have to buy a new set of tires, but will if this will potentially affect the car's health and my warranty. I'm working on getting an accurate measurement of the tires. Any advice there is also helpful. Thanks!

 

this comes up ALL THE TIME. the answer: it depends on who you choose to listen to. my tires have almost 29k miles on them. if i needed to replace a tire i would replace all four. if i had 20k miles on them i would also replace all four

it's not worth the risk to me to squeeze out a few extra bucks

 

THEN AGAIN...

This is a head scratcher for me. I can't find even ONE story online of a Subaru AWD being destroyed let alone destroyed because of the tires. And I find it hard to believe that most Subaru owners are making sure that all four tires on their car have the same circumference. So I have to conclude that there is more wiggle room when it comes to this than the owner's manual would have us believe.

Are YOU aware of anyone's AWD being destroyed on a Subaru because of tire circumference?

[I wrote this in reply to another topic thread]

 

I've been on walkabout and it's good to see we're still arguing about this, LOL. All four tyres/tires should be the exact same brand and type and very close in wear (why most professional tyre shops recommend shaving a new tyre to match what's on the car). Would you rather save $100 and have to replace the CVT?

 

I will ask you directly...

Are YOU aware of anyone's AWD [or CVT for that matter] being destroyed on a Subaru because of tire circumference?

 

Wow, that escalated quickly. Just get your tires checked by a professional, dude. WTF?

 

XXXX

 

It’s too bad this thread got pushed into the gutter yesterday, because it poses a valid question. I think we’d all agree that tires need to be matched for handling and safety reasons, and that there’s a tipping point where a level of tire mismatch has the potential to cause damage to an AWD vehicle, but in real world terms, I haven’t seen an analysis of exactly where that tipping point is.

I think it’s reasonable to assume that Subaru’s recommendations on this are pretty conservative, but it’s also reasonable to treat this as a situation where it’s better to be safe than sorry.

 

This isn't only a Subaru issue, it applies to all AWD vehicles. This article explains why and indicates that besides having the exact same tires all around that they should be within 2/32" of each other, hence shaving down a replacement tire to match.

https://www.anrdoezrs.net/click-100...5734228?sid=autolinker&url=https://www.tirerack.com/tires/tiretech/techpage.jsp

 

This is what my local indy mechanic says as well. When I buy new tires, I buy a set of 5 and use the spare in the rotation. That way, if a tire becomes unrepairable, I still have 4 equally worn tires.

@CanadaCraig does bring up a valid point and I have yet to hear of a viscous coupler failure as a result of unmatched tires. However, as others have said, I'd rather be safe than sorry.

This does bring up another issue. The compact spare is WAAAAAY different in diameter than the difference between the same size tires with different levels of wear. The question I have is how long can you drive on your compact spare before you trash your AWD system?

 

What does the manual say? I doubt you could go 300 miles safely.

 

It might be worth mentioning that the cvt does not use a viscous coupler, it uses clutch packs to send power front and rear. The manual tranny has the viscous differential.

 

Given that... would all four tires being the same circumference be more important on a Crosstrek with a manual transmission? And do you think the tires would be less or more of a threat to the transmission itself? [Compared to the impact they might have on a CVT]

 

Paraphrasing fron another thread, I see no worries at all about the centre differential. The math just don't seem to support any reason it would be subject to excessive strain.

A 3/32" difference in the radius of a tyre (13.8") corresponds to 0.65%. At 75 mph with 730 revs/mile, the normal wheels rotate 15.208 times per second.

With one tire smaller, the difference in the wheel radii will make it rotate 0.65% faster, which adds an extra 0.099 rotations per second... or one full extra turn per 10 seconds (6 rpm).

You now have the rear or front open diff accommodating a 6rpm difference between L and R wheels, which translates to +3rpm at the shaft. That means that the center diff will be subject to a 3 rpm (1 rotation every 20 s) discrepancy between front and rear. A viscous-coupled diff won't even notice it as the resulting friction won't generate enough heat to melt a snowflake. It's harder to analyze the CVT' multi-clutch differential's susceptibility to continous rotational differences, but again.. we're analyzing the impact of 3rpm. Safe to assume it's negligible, because...

...last but not least: tire deformation. Under full cargo load or with a trailer, the rear is squished. Enough to reduce the effective radius of both properly inflated rear tires by a few /32s. Which, in turns, adds an n-times larger rotational speed discrepancy at the center diff.

I just can't see any negative impact on the transmission. Handling, stability, VDC yes.. but not on the mechanical side.

 

...

I just can't see any negative impact on the transmission. Handling, stability, VDC yes.. but not on the mechanical side.

Math

 

I don't have a CVT but with auto cars Subaru use to have a spot to put a fuse in to put it in FWD mode when you got a flat. I don't think that is the case now but Subaru still requires the temp spare to be put on the rear though, not sure if most people are aware of that.

I've seen first hand what mismatched tires can do, I've mentioned it here before, it was a 4EAT though not a CVT. The transmission developed a shudder when taking off from a stop and on turns, with two new tires in the front and old worn ones in the rear. It was driven almost a week like this and the shuddering got progressively worse. I did not measure the difference but it did stop shuddering when they got tires on the rear that matched. So in this case no immediate damage but I have no idea about long term. Wasn't my car.

From the owners manual :

From an older Subaru with a 4EAT auto:

 

I don't have a CVT but with auto cars Subaru use to have a spot to put a fuse in to put it in FWD mode when you got a flat. I don't think that is the case now but Subaru still requires the temp spare to be put on the rear though, not sure if most people are aware of that.

I've seen first hand what mismatched tires can do, I've mentioned it here before, it was a 4EAT though not a CVT. The transmission developed a shudder when taking off from a stop and on turns, with two new tires in the front and old worn ones in the rear. It was driven almost a week like this and the shuddering got progressively worse. I did not measure the difference but it did stop shuddering when they got tires on the rear that matched. So in this case no immediate damage but I have no idea about long term. Wasn't my car.

From the owners manual :

View attachment 314590

From an older Subaru with a 4EAT auto:

View attachment 314591

The special fuse to make your car FWD was pre-2010, before CVT's and before the AWD system became electronic. It was only on 4EAT cars.

My guess the reason they say not to use the temp spare on the front is that it can affect steering and make the handling unsafe. Interestingly, there is nothing there that states how long a temp spare should be used other than the life of the temp spare. As I stated before, the difference in circumference between a normal and compact spare is WAAAAAAAY more than the difference between a new and worn normal tire. One has to wonder.

 

I'm going to keep looking into this. For one thing... I want to find out if the threat is bigger to a Crosstrek with a CVT or with a manual transmission. And I'd like to find more first hand experiences from people who have experienced this. [In addition to the one you gave us peaty] I find it all quite interesting. And a bit puzzling to say the least. There is one story online about a guy with a CVT Impreza. His transmission was destroyed by uneven tires [according to the Subaru dealer he took it to] and the dealer said it was no longer covered by the warranty. But as the story goes... he put two brand new tires on back and the orginal, very well worn rear tires on the front. I imagine he drove it that way for quite awhile.

 

I'm going to keep looking into this. For one thing... I want to find out if the threat is bigger to a There is one story online about a guy with a CVT Impreza. His transmission was destroyed by uneven tires [according to the Subaru dealer he took it to] and the dealer said it was no longer covered by the warranty. But as the story goes... he put two brand new tires on back and the orginal, very well worn rear tires on the front. I imagine he drove it that way for quite awhile.

Was it the transmission that was destroyed or was it the transfer case? I suspect the transfer case.

No idea. I'm not an engineer. Honestly, all this reasoning and napkin math is cool, but I'm still not going to risk my drive train on it. It would be nice (and cheaper) if I could just replace a single tire, or even just match axel pairs, but the OEM is very clear about not doing that.

If you have to buy a new tire because of an unrepairable road hazard, you can have to new tire shaved to match more closely.

 

Here's more of "napkin math". The spare is 145/80R17. That's a whooping 5.4% in radius/circumference decrease w.r.t. stock size. As a reminder, a 3/32" tread depth difference is equivalent to 0.68%. Running the spare is basically like running a tire with 24/32 (!) less tread depth.

The owner's manual allows continuous driving up with 1 spare wheel at speeds up to 50mph. At 50 mph with 730 revs/mile, normal wheels rotate 10.1 times per second. The spare will rotate 5.4% faster, namely +0.55 rotations/second or +33rpm - and it's within allowed specs

So, how can a 33 rpm rotational speed difference be perfectly acceptable for the center diff, autorized by the owner's manual, but a 6 rpm difference (corresponding to a 3/32" tread depth difference at 75mph) would be detrimental ?

Note that the owner's manual does not include any warning about avoiding driving for too long with a spare. Just a limitation on the top speed (which does drive the wheel RPM difference, but could very well be justified by degraded braking and handling abilities). If that's not a pretty rock solid argument that the drivetrain is immune to tread depth mismatches, then nothing else is..

 

I think the difference here is that you are never running a compact spare for more than its life which is only 200-300 miles.

Consider a set of tires with a life of about 40K miles. You trash one of them at 20K miles and replace with a new one. So you are then running the mismatched set for another 20K miles. Note that you won't wreck your AWD system in a NY minute, but you may shorten its life by..................who knows? Maybe 20K miles? So instead of wearing out at 200K miles, you may wear it out at 180K miles? Maybe 50K miles? So then it would wear out at 150K miles instead of 200K miles?

All of this is just speculation. My point is the term "premature failure" is relative. To be clear, most of my cars die of rust before much else. That may change though now that I have a two car garage.

 

Agreed. We just don't know whether potential differential damage is instantaneous (i.e. due to heat/strain induced by real-time slippage or rpm difference) or cumulative (included by fatigue or wear, accelerated by a rotational mismatch). With a manual and the viscous-coupled differential, I'm quite certain there is no cumulative wear due to its construction - there no friction or contact in between solid plates at all. With the multi clutch pack in the CVT, I'm less certain (friction-induced rotational abrasion is cumulative over the total number of turns). Interesting discussion... and it sucks that the Crosstrek doesn't have a full-sized spare.

 

I'm inclined to think it's cumulative rather than instantaneous. Sort of like not changing your oil in a timely manner.

I believe the cumulative damage is worse with the viscous coupler than with the system with the CVT - at least that's what the service manager at my old dealership told me.

 

For those of us who don't drive the average annual mileage, remember that rubber ages and while you may have a lot of tread depth the tire could still be dangerous at highway speeds. And while writing this I realized I haven't checked the age of the tires on my new to me 25yo car...

 

Dry rot can indeed be an issue when time far exceeds miles. Though if you have an attached garage, that happens much more slowly.

Examine carefully for cracking.

 

We had a 97 Outback limited auto that came with Michelin MX4 tires (I think it was) the tires just seem to stop wearing at some point. There was a fair amount of tread left but eventually the rubber got so hard the car would lose traction in the wet easily. I guess the only visual exam, aside from dry rot, is to look at the DOT date, depending on where you look 6 years they say but 10 years max? I suppose it depends on where you live and park the car though. Even Discount Tire says there is not exact way to say when they go bad, lots of things to consider I guess >>Link<<

 

I remember the Michelin MX4's well. They were on my 97 Outback when I bought it. Since the car had 61K miles, they were well worn and needed to be replaced. I replaced them with BF Goodrich Long Trail T/A's. Those lasted until I traded the car in at 112K miles. The Long Trail T/A's were great tires. They took whatever conditions you threw at them and were even pretty decent in snow. Unfortunately, my 09 Outback took a different size and I couldn't find Long Trail T/A's in that size when it came time for new tires on that car.

 

There are thousands of old (and not so old) Subarus still being driven that do not have their tyres replaced in 4's...and I have never heard of an actual problem

 

Thanks for all the messages. I went ahead and bought 4 new tires. Didn't want to risk it. I sort of knew that was the way, but figured I'd check here, too. Appreciate it. The only tire shop within 1.5 hrs of me is really bad, but so far, it's all worked out alright. Thanks again!

 

This week a friend had a blowout a right rear tire in her Legacy. She initially called me because she remembered me saying something about tires with different sizes being bad and possibly needing 4 tires. Turns out she was waiting for AAA on the side of a very busy highway to come and change the tire. She was there about 2+ hrs. It wasn't that far from me. I got there first and changed it for her. Since the blowout was on the rear no need to swap tires around. She only drove home, maybe 5 miles then to the Subaru dealer where she got the tires originally, another maybe 2 miles. The tires had ~34K miles and were within the road hazard timeframe of 2 years. The dealer said if she wanted only one tire (not something she asked) they would replace for free but she would also sign a waiver in the event of drivetrain damage. She ended up getting 4 new tires but paying for 3.

Info. on the subject from Mr Subaru (skip to 7 min for info on diff sizes)

I'm attaching scans from Subarus end wrench mag they use to send to the dealer service departments from three different issues and a statement from subaru on the subject.

Tire Wear/Rotation/Replacement/Temporary Spare Tire Operation We all know that tires wear. There are certain things you, as an owner, can do to help minimize this wear. 1. Keep your tires properly inflated. This is perhaps the single most important thing you can do. Information on correct tire inflation pressure is provided in your owner's manual and on a placard on the driver's door pillar. How often you check your tire inflation is up to you. As a rule of thumb, tires should be checked every time you fill your car with gas. Remember that tires should be checked when cold since tire pressure will increase as the tires warm up. 2. Rotate your tires regularly. SUBARU recommends tires be rotated every 7,500 miles. These are recommended intervals only. Your actual driving conditions may warrant more frequent rotation. Factors such as road surfaces, driving techniques/habits, vehicle loading, and weather, to name just a few, can all have an effect on tire wear. 3. When replacing tires, it is always recommended that you choose tires of the same type, size, construction, and manufacturer as those present on the vehicle as original equipment. On All-Wheel Drive (AWD) vehicles, it is extremely important that the rolling or outer circumferences of the tires be within 1/4 inch of each other. This means that you must physically measure the size of the tire. This is best done with the weight of the vehicle off the tire and at the tread centerline. Also be certain that the tires are properly inflated since this can affect your readings. If you need to replace just one tire, the same holds true. The measured difference in circumference between the replacement tire and the other tires on the vehicle cannot exceed 1/4 inch. Depending on the vehicle mileage, it might be better to replace all four tires. If the vehicle mileage is low and the tires have been rotated and driven at the proper inflation pressures, then you may be within the allowable 1/4-inch difference in circumference. If the difference between the new/replacement tire and the current tires is within this 1/4-nch maximum, then the tire should be compatible. If it is not, then other tires will need to be replaced to conform to the 1/4-inch rule. Exceeding this 1/4-inch difference in tire circumference can place unnecessary wear on drivetrain components, possibly causing them to wear out prematurely. If your vehicle is AWD (All-Wheel Drive) with an automatic transmission and the temporary spare tire has been installed, put a spare fuse (15 amp) inside the FWD (Front-Wheel Drive) fuse holder (refer to your owner's manual for location). Confirm that the Front-Wheel Drive (FWD) warning light comes on before you drive the car. Installation of this fuse deactivates AWD to prevent possible damage to the drivetrain components that can result due to the use of the smaller temporary spare tire. After re-installing the conventional tire, remove the fuse from the FWD fuse holder to restore AWD operation. For more information on tires and temporary spare tire usage, always consult your Subaru owner's manual.

 

I appreciate all the arm chair engineering and what seems to be solid math involved. For me though, if I am on used tires and have 1 bad one, I will replace all 4. It is just not for me worth the risk of long term damage/wear on the driveline. Over cautious, waste of money or whatever it is, that is just the cost of owning an awd in my mind. Having had my cherokee now for many years, tires were always in pairs at minimum anyways but better with a full set. I see it as a good chance to get fresh rubber under me, the single most important part of the vehicle, contact to the ground can never be to New for me and no downside to 4 brand new tires. Plus, you can always find someone willing to buy your used tires to help offset the cost of new tires if needed. I just wish we had a Full Size spare 🤯.
Topics like this can never be settled but I appreciate they exist to help people make their own choices.

 

I appreciate all the arm chair engineering and what seems to be solid math involved. For me though, if I am on used tires and have 1 bad one, I will replace all 4. It is just not for me worth the risk of long term damage/wear on the driveline. Over cautious, waste of money or whatever it is, that is just the cost of owning an awd in my mind. Having had my cherokee now for many years, tires were always in pairs at minimum anyways but better with a full set. I see it as a good chance to get fresh rubber under me, the single most important part of the vehicle, contact to the ground can never be to New for me and no downside to 4 brand new tires.

Or you can buy tires in sets of 5 and use the spare in the rotation. Then if one tire becomes unusable, you still have 4.

What I would like to know is at what point of wear should a new tire not be added to the used 3? Most new tires have a tread depth of 10/32" to 12/32". The law states that 2/32" or less needs to be replaced. Common sense says 4/32" is as shallow as you should go. For snow tires, that minimum recommended tread depth is 6/32".

Plus, you can always find someone willing to buy your used tires to help offset the cost of new tires if needed.

But who would want to buy a set of 3 used tires?

 

According to the video above from Mr. Subaru @ just before 8 min in he says all tires need to be within 2 - 3 - / 32nds of each other front to back side to side.

I agree the least I'd like to drive around it is 4/32nds on regular street tires.

 

According to the video above from Mr. Subaru @ just before 8 min in he says all tires need to be within 2 - 3 - / 32nds of each other front to back side to side.

I agree the least I'd like to drive around it is 4/32nds on regular street tires.

I just found this from Tirerack. They say Subaru's recommendation is no more than 2/32" difference:

Manufacturers Recommendations
Audi	As published in their vehicle owner's manual, "rolling radius of all four tires must remain the same" or within 4/32-inch of each other in remaining tread depth.
Porsche	Cayenne within 30% of the other tire on the same axle's remaining treadwear.
Subaru	Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

https://www.anrdoezrs.net/click-100...=autolinker&url=https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=259

 

Subaru once told me that if the tires have 15k miles or less you can get away with replacing one or however many need to be replaced nd not have to buy all 4. Anything more than 15k miles you have to replace all 4. FWIW