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Discussion Starter · #1 ·
I hit some road hazard and one tire blew out. The dealer recommends changing all four tires. I know the manual recommends that, but my Ascent only has 9,000 miles on it, so the wear on the tires are pretty minimal. At the time, I refused and just replaced the one. After that, I've seen a few posts on the internet about how the tires have to exactly the same to each other, so I am little worried. Should I have replaced all four tires? Thank you so much for your help.
 

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So the other 3 tires are at 7/32, the new one 10/32. The new one is the left rear one, if I might add. Should I replace all 4 or have the new one shaved down to 7/32?
 

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My opinion is just that my opinion.....

3/32 sounds like more than minimal wear. I would replace them all at that point. With one taller tire it will not only cause more slippage in the diff side to side but also front to back.

I sure others will chime in too. I am interested in hearing what they say.
 

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If I were in your situation, I would NOT change all four. If one wanted to give in to their OCD, they could "shave" the tire, as someone mentioned above - but I would not have done that either. Think of how many tens of thousands of shoulder or sidewall blowouts or punctures there are every day. Guess how many get 4 new tires when that happens? Sounds like the dealer that recommended that was trying to take advantage. Just my .02 cents.
 

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It boils down to your budget. If you just replace one, yes it is necessary to shave it down to match the contralateral tire. Otherwise there will be accelerated wear to your differential, since you have awd.

If budget is not an issue, then I'd replace them all with a new set of good all terrain tires like the Continental Terrain Contacts.
 

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You haven't stated so, but I assume the blown-out tire is not repairable? If this is the case, replace it, have it shaved to match the others, and be done with it.
 

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If I were in your situation, I would NOT change all four. If one wanted to give in to their OCD, they could "shave" the tire, as someone mentioned above - but I would not have done that either.
Perhaps you'd feel differently when, not if, the size discrepancy damaged your drivetrain? That's the reason for the need to replace all four (or shave a new one) if there is more than a slight difference in diameter. This isn't a "Subaru" thing, either. It's a requirement on pretty much all 4WD/AWD vehicles. The spare has to match, too...which is one of the things that folks up-sizing tires sometimes forget, both functionally and relative to the added cost to their up-sizing project.
 
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I don't suppose you are located in central lower Michigan area? I have four 20" OEM falkens with 1000 miles on them... I have the wheels, too.
PM me if interested in them and can arrange for pick up.
 

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So the other 3 tires are at 7/32, the new one 10/32. The new one is the left rear one, if I might add. Should I replace all 4 or have the new one shaved down to 7/32?
If I were in your situation, I would NOT change all four. If one wanted to give in to their OCD, they could "shave" the tire, as someone mentioned above - but I would not have done that either. Think of how many tens of thousands of shoulder or sidewall blowouts or punctures there are every day. Guess how many get 4 new tires when that happens? Sounds like the dealer that recommended that was trying to take advantage. Just my .02 cents.
Subaru AWD is different than part time systems, and the tread difference is below the limit.

Subaru specs are:
  • 1/4" total circumference difference or less
  • or 2/32" tread difference or less.
A differential is far more expensive than a set of tires, and then there's the labor.
 

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It's times like these when you wish car manufacturers would still offer a full-size spare. We just had to change all four tires on my wife's 2018 Outback, because they were down to 6/32" on all tires, when it had a nail in the sidewall. Another additional/unexpected expense for owning an AWD vehicle.
 

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It's times like these when you wish car manufacturers would still offer a full-size spare.
Fortunately, Subaru has been listening. Certain Outback trims now once again come with a full sized spare. I expect that the Ascent will need some design mods to fit one properly, but, with enough owner feedback, I expect we'll either see a full spare option, or at least the ability to fit one.

I am crossing my fingers. A full sized spare causes issues off roading with the Ascent.
 

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If you put on a full size spare, that is most likely going to be off more than the 2/32" with the other tires by the time the spare is needed? Is it OK running different sized tires for a short period of time, or does Subaru install spares that are equivalent in OD to a slightly worn tire?
 

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Discussion Starter · #14 ·
I don't suppose you are located in central lower Michigan area? I have four 20" OEM falkens with 1000 miles on them... I have the wheels, too.
PM me if interested in them and can arrange for pick up.
I am in California. Thanks for offering though.
 

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If you put on a full size spare, that is most likely going to be off more than the 2/32" with the other tires by the time the spare is needed? Is it OK running different sized tires for a short period of time, or does Subaru install spares that are equivalent in OD to a slightly worn tire?
Not just should it be used for a very limited time, but the manual implies the warning of how driving on it affects our always on AWD... here's some of the places from the manual that discuss using a donut wheel (aka "temporary spare tire") - I'm bored on lunch break, so, I went a little overboard: ;) :ROFLMAO:

It's likely affecting AWD and making the system complain - Page 182:
If the temporary spare tire is used, the AWD warning light may flash. Use of the temporary spare tire should therefore be restricted to the minimum time necessary. Replace the temporary spare tire with a conventional tire as soon as possible.
VDC May also not operate properly (and you may be surprised to know how many times it does things) - Page 336:
Use only the spare tire that is shown on the tire placard to replace a flat tire. With a temporary spare tire, the effectiveness of the Vehicle Dynamics Control system is reduced and this should be taken into account when driving the vehicle in such a condition.
It is specifically sized for temporary use, while other sizes may result in severe mechanical damage to the drivetrain - Page 375:
If you use a temporary spare tire to replace a flat tire, be sure to use the original temporary spare tire stored in the vehicle. Using other sizes may result in severe mechanical damage to the drivetrain of your vehicle.
And on Page 405:
The temporary spare tire is smaller and lighter than a conventional tire and is designed for emergency use only. Remove the temporary spare tire and reinstall the conventional tire as soon as possible because the spare tire is designed only for temporary use.
On Page 405-406 are the very important notes for temporary spare tire usage - at least one of which most people don't know (the last one):
When using the temporary spare tire, note the following:
  • Drive with caution when the temporary spare tire is installed. Avoid hard acceleration and braking, or fast cornering, as control of the vehicle may be lost.
  • Do not exceed 50 mph (80 km/h).
  • Do not put a tire chain on the temporary spare tire. Because of the smaller tire size, a tire chain will not fit properly.
  • Do not use two or more temporary spare tires at the same time.
  • Do not drive over obstacles. This tire has a smaller diameter, so road clearance is reduced.
  • When the wear indicator appears on the tread, replace the tire.
  • The temporary spare tire must be used only on a rear wheel. If a front wheel tire gets punctured, replace the wheel with a rear wheel and install the temporary spare tire in place of the removed rear wheel.
Oh, and by the way, for those who don't know how, here's the trick if you get a flat in the front.
  1. Take out your spare tire.
  2. Install it on a REAR wheel.
  3. Take that full sized rear wheel and install it where the flat is.
  4. Put the flat, fully deflated, back in the spare tire storage area, facing UP.
And, here's the User Manual, for anyone who's bored and wants to read more:
 

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...
Oh, and by the way, for those who don't know how, here's the trick if you get a flat in the front.
  1. Take out your spare tire.
  2. Install it on a REAR wheel.
  3. Take that full sized rear wheel and install it where the flat is.
  4. Put the flat, fully deflated, back in the spare tire storage area, facing UP.
And, here's the User Manual, for anyone who's bored and wants to read more:
I doubt many people know about this or are willing to spend the time along the side of the road replacing both the front and rear tires. This is dangerous advice from Subaru. You're in significant danger working on the side of a road. Your life is worth far more than any potential damage to any car. You don't want to spend a minute more there than necessary.

Take a look at these videos. Unfortunately, this is not even that unusual, it happens a lot. Just search "cop hit by car" on Youtube and you'll be amazed at how often this happens.

Warning: Graphic content. All persons shown survived, but there are many incidents much worse.




I had a flat on the highway near Washington DC. I was almost hit twice while trying to repair it. I was so shook up afterward, I could hardly drive. Also, I once stopped to help someone who drove off the highway into a ditch. Again I was almost hit more than once.

Parking and working on your car alongside a highway may be one of the most dangerous things you'll ever do. My advice is to do your best to get your car running again asap and then get the hell out of there. You can always address the issue once you reach safety.
 

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I doubt many people know about this or are willing to spend the time along the side of the road replacing both the front and rear tires. This is dangerous advice from Subaru. You're in significant danger working on the side of a road. Your life is worth far more than any potential damage to any car
Don't worry, Subaru thought of that too. It's literally one of the first things in that section (spare tire section) of the instructions. They expect people will pull off the road before trying to change a tire, and recommend such. ;)

  • The hazard warning flasher should be used in day or night to warn other drivers when you have to park your vehicle under emergency conditions.
  • Avoid stopping on the road. It is best to safely pull off the road if a problem occurs.
  • The hazard warning flasher can be activated regardless of the ignition switch position. Turn on the hazard warning by pushing the hazard warning flasher switch. Turn it off by pushing the switch again.
I should have included it. Thanks for pointing out the importance. The deficiency was on my part, not Subaru's.
 

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I hit some road hazard and one tire blew out. The dealer recommends changing all four tires. I know the manual recommends that, but my Ascent only has 9,000 miles on it, so the wear on the tires are pretty minimal. At the time, I refused and just replaced the one. After that, I've seen a few posts on the internet about how the tires have to exactly the same to each other, so I am little worried. Should I have replaced all four tires? Thank you so much for your help.
Similar situation in California & preparing for a 12 hour drive. We had around 12k miles on the car. Found a dealer in Fairfield that okayed the replacement of one OEM tire. All's been well since and after multiple long trips with and without a trailer. I've not noticed any ill effects and I'm pretty picky on handling.
 

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If you put on a full size spare, that is most likely going to be off more than the 2/32" with the other tires by the time the spare is needed? Is it OK running different sized tires for a short period of time, or does Subaru install spares that are equivalent in OD to a slightly worn tire?
If you have a full size spare, it should be rotated in with your other tires whenever they are rotated which will spread out the wear. Or at least you do that on a standard vehicle with a full sized spare. Now I wonder if it's different for full time AWD? So few cars have a full-sized spare these days!
 
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"Full size" spare doesn't always mean an identical tire to what's on the four corners normally...in some vehicles, it's a normal function tire of the same diameter and load rating, but narrower so it fits in the compartment. That's how Grand Cherokee is equipped for vehicles that have Factory Towing, for example. The wheel is a simple steel wheel. You can run that spare full time if you need to including when towing up to the vehicle rating.
 
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