replacing tires on AWD cars do you have to do all 4

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3feetpete
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replacing tires on AWD cars do you have to do all 4

Post by 3feetpete » Wed Feb 06, 2019 7:24 pm

Do you really have to replace all four tires on an AWD car if only one is bad. My wifes 2006 Subaru Outback got a nail in the sidewall. Two different places recommended I replace all four saying that to replace one or even two at a time would potentially damage the transmission. Sounds like BS to me but I went online and that seems to be the consensus. I thought I'd check with my trusty Bogleheads.

TheDoorMan19
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Re: replacing tires on AWD cars do you have to do all 4

Post by TheDoorMan19 » Wed Feb 06, 2019 7:29 pm

I asked my Acura dealer and they said no and replaced just one. I figured if they didn’t try to rob me when I was willing, it really must not be necessary. But I know nothing at all about cars.

onourway
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Re: replacing tires on AWD cars do you have to do all 4

Post by onourway » Wed Feb 06, 2019 7:31 pm

Yes, with your Subaru you should do this. Ask your mechanic to order one tire from Tire Rack shaved to the depth of the remaining three tires. They charge about $15 for this service.

arf30
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Re: replacing tires on AWD cars do you have to do all 4

Post by arf30 » Wed Feb 06, 2019 7:31 pm

You're supposed to replace all 4 to avoid wear on the differentials. You can also buy one new tire and have it "shaved" down to the correct diameter but I've never done this.
Last edited by arf30 on Wed Feb 06, 2019 7:32 pm, edited 2 times in total.

arrieros81
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Re: replacing tires on AWD cars do you have to do all 4

Post by arrieros81 » Wed Feb 06, 2019 7:32 pm

One of the reasons I don’t want to own another AWD.

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Nicolas
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Re: replacing tires on AWD cars do you have to do all 4

Post by Nicolas » Wed Feb 06, 2019 7:42 pm

I also own a 2006 Outback. My independent mechanic replaced one of my tires last autumn, he didn’t say anything about replacing all four and I didn’t think to ask so only that one was replaced. So far, so good. We will trade this car in for a newer one about a year from now, hope it lasts that long.
GATHER ye rosebuds while ye may, Old Time is still a-flying: And this same flower that smiles to-day To-morrow will be dying.-- Robt. Herrick, 1591–1674, from To the Virgins, to make much of Time

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Re: replacing tires on AWD cars do you have to do all 4

Post by btenny » Wed Feb 06, 2019 7:43 pm

Yes you have to replace all four tires to get them matched. If they are not matched in type and size and diameter (which changes with wear) they will cause the transfer case and differentials to break.

How many miles are on the tires? On the spare? Are the tires and spare matched? If you had three new tires and one with a nail and a new spare then you could put the matching spare on the ground. Otherwise you have to match the tires you put on the ground.

You might go to a tire shop like Discount tire and ask them if they have some new car take offs. Many time new car owners change tires and sell the take offs. You can then buy the set at a discount.

Good Luck.

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Nicolas
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Re: replacing tires on AWD cars do you have to do all 4

Post by Nicolas » Wed Feb 06, 2019 7:45 pm

btenny wrote:
Wed Feb 06, 2019 7:43 pm
Yes you have to replace all four tires to get them matched. If they are not matched in size and diameter (which changes with wear) they will cause the transfer case and differentials to break.

How many miles are on the tires? On the spare? Are the tires and spare matched? If you had three new tires and one with a nail and a new spare then you could put the matching spare on the ground. Otherwise you have to match the tires you put on the ground.

You might go to a tire shop like Discount tire and ask them if they have some new car take offs. Many time new car owners change tires and sell the take offs. You can then buy the set at a discount.

Good Luck.
The spare tire on a 2006 Outback is a thin tire meant only for temporary use. It’s not the same size as the others. That’s how it is on mine, anyway.
GATHER ye rosebuds while ye may, Old Time is still a-flying: And this same flower that smiles to-day To-morrow will be dying.-- Robt. Herrick, 1591–1674, from To the Virgins, to make much of Time

mhalley
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Re: replacing tires on AWD cars do you have to do all 4

Post by mhalley » Wed Feb 06, 2019 7:51 pm

I think there is a size differential that needs to be met, so it depends on how much wear is on the old tires. Subaru’s have the least tolerance per this article:
. Subarus: 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. Therefore the 2/32nds rule is only good if you have the exact same size, brand, and model tire. The other way to check is to measure the tire around the circumference and then the tolerance is 1/4". That was circumference, not diameter, so you have to have a flexible tape ruler and measure around the tread of the tire. Also, having air in the tire affects the circumference. Though the change due to having air in it is only about 1/8", it's pretty critical since the tolerance is only 1/4". Therefore, to get an accurate measurement, it's necessary to let the air out before you measure (since the prospective replacement probably won't be aired up). This second method allows you to check compatibility with any make or model of tire. When we were almost finished with this article, a Subaru came in that needed only one tire, so the picture shows us measuring the prospective replacement tire with our "Subaru Tool".


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. Granted it won't happen overnight, and the AWD on many Subarus can be disabled, but the transmissions can definitely get ruined and then they have to be repaired or replaced. Then the cost will be a lot more than the cost of tires!

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Re: replacing tires on AWD cars do you have to do all 4

Post by Starfish » Wed Feb 06, 2019 7:52 pm

btenny wrote:
Wed Feb 06, 2019 7:43 pm
Yes you have to replace all four tires to get them matched. If they are not matched in type and size and diameter (which changes with wear) they will cause the transfer case and differentials to break.
So how do you match the tire pressure which should affect the effective radius more than 1 mm of rubber?

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3feetpete
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Re: replacing tires on AWD cars do you have to do all 4

Post by 3feetpete » Wed Feb 06, 2019 7:59 pm

Thanks for all the prompt responses. That's a very good point about the tire pressure affecting the diameter more than the tread wear. And for that matter the different wear between the front and the back can be significant as well. I think it's 90% BS but the owners manual says so and I went ahead and bought all four. If it happens to me again I'll look into the shaving.

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GMCZ71
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Re: replacing tires on AWD cars do you have to do all 4

Post by GMCZ71 » Wed Feb 06, 2019 8:05 pm

You should also account for age of all the tires. The safety recommendation is 6 yrs to 10 yrs on premium brands then the tires should be replaced.
John

ThatGuy
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Re: replacing tires on AWD cars do you have to do all 4

Post by ThatGuy » Wed Feb 06, 2019 8:10 pm

Starfish wrote:
Wed Feb 06, 2019 7:52 pm
btenny wrote:
Wed Feb 06, 2019 7:43 pm
Yes you have to replace all four tires to get them matched. If they are not matched in type and size and diameter (which changes with wear) they will cause the transfer case and differentials to break.
So how do you match the tire pressure which should affect the effective radius more than 1 mm of rubber?
What is the manufacturing tolerance on a consumable like this anyways?
Work is the curse of the drinking class - Oscar Wilde

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Re: replacing tires on AWD cars do you have to do all 4

Post by Spirit Rider » Wed Feb 06, 2019 8:14 pm

onourway wrote:
Wed Feb 06, 2019 7:31 pm
Yes, with your Subaru you should do this. Ask your mechanic to order one tire from Tire Rack shaved to the depth of the remaining three tires. They charge about $15 for this service.
Most AWD car manufactures have a specification on how close the tread depth should match on all four (4 tires). Subaru's specification for this is 2/32", it might even be in your owners manual. It is also necessary to match the exact same tire.

However, the first question is; what is the tread depth on the three remaining tires? If they are close to end of life, replace all four, but if they are fairly new.

Measure the tread depth of the other three tires, average and round up to the nearest 32nd. Then go to Tire Rack; order the exact same tire, have it shaved to the desired depth, delivered and installed locally. $15 - $20 to have a tire shaved is far better than putting unnecessary strain on the AWD drive train.

We have massive frost heaves and pot holes, some of which hide at night. I end up having to replace a tire every few years. When I replace all four, I always save the two best tires. You don't really want to be buying a new tire and having it shaved way down when there isn't much tread left.

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Re: replacing tires on AWD cars do you have to do all 4

Post by The Wizard » Wed Feb 06, 2019 8:20 pm

Subaru takes all the fun out of having differentials...
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livesoft
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Re: replacing tires on AWD cars do you have to do all 4

Post by livesoft » Wed Feb 06, 2019 8:24 pm

Starfish wrote:
Wed Feb 06, 2019 7:52 pm
So how do you match the tire pressure which should affect the effective radius more than 1 mm of rubber?
One uses a tire pressure gauge. If the car is put up on a rack with the wheels hanging, then one can measure the circumferences of all the tires. One can even change the air pressure in the tires and make multiple measurements. Then plot Pressure vs Circumference.

I would guess that a 5 psi difference would not make much difference. Inside the door well of my Subaru, the recommended tire pressures are different by 2 PSI for the front and rear tires. However, the dealership so far has always put the same PSI in all 4 tires and always 2 to 3 PSI lower than what is on the sticker in the door well. Since the car has tire pressure monitoring system, one can see the tire pressures vary depending on ambient temperature, elevation, which side of the car was sitting in strong sunlight, and driving time. The TPMS matches what I measure with a handheld tire pressure gauge.
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dknightd
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Re: replacing tires on AWD cars do you have to do all 4

Post by dknightd » Wed Feb 06, 2019 8:51 pm

It really depends on what you want for your wife's car. Or what she wants.
Minimal is four tires that hold air.
Better is tires balanced on each side.
Best is 4 matching tires

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Socrates28
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depends on wear of other 3 tires

Post by Socrates28 » Wed Feb 06, 2019 8:58 pm

if a lot of tread life just change one, if moderate or less I'd change all four

on my front wheel drive (on demand 4wd Honda) the front tires were worn and back were ok.....I changed all four

Dontridetheindexdown
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DIFFERENTIALS & TRANSFER CASE

Post by Dontridetheindexdown » Wed Feb 06, 2019 9:00 pm

For what it's worth:

1. Left and right tires on each axle turn at different speeds. That is why we have a "differential" on each drive axle. If you remove the differential cover, you will see a spider gear (4 helical-cut gears) that allow the left and right wheels to turn at at different speeds, including one side not turning at all, or even turning backwards. Without differential gears, you would not be able to turn a corner on paved roads without scrubbing your tires or skidding. Whether you are speeding down the interstate, or just meandering down a dirt road, the left and right tires on each axle are always turning at different speeds.

2. With all wheel drive, there is a third differential in the transfer case. This allows the front and rear axles to turn at different speeds. Open up the transfer case and look at the third differential if you do not believe me. Old-style 4 wheel drive vehicles did not have a differential in the transfer case. That worked in off-road conditions (sand, snow, or mud). On paved roads, the old-style 4 wheel drive vehicles exhibited serious handling problems, especially at highway speed, and especially on paved roads.

3. All wheel drive vehicles use traction control (automatic braking system (ABS) sensors/actuators to control "slip," and/or limited slip differentials) to moderate the differentiation between left and right tires on both front and rear axles, and also to moderate the differentiation between front and rear axles. The best modern vehicles will "cruise control" at high speed on slick roads, continually modifying the differentiation between tires on the same axle, and between the front and rear axles.

4. There is no reason whatsoever to change all 4 tires when one tire needs replacement. All 4 tires rotate at speeds independent of each other, no matter what type of vehicle you drive. The crown of the road, the amount of debris on the road, and the fact that all vehicles turn left and right, require that each tire turns independently, at its own speed.

5. Feel free to discount my input. I have more than 50 years experience in fleet maintenance, including tactical military and emergency vehicles. Do any of you believe that emergency vehicles get all 6 or all 10 tires replaced when one tire fails?

6. Feel free to do whatever you wish. I know for a fact there is no way whatsoever that you can damage a differential, a transfer case, or a transmission by replacing just one tire.

7. Tire shaving is beyond ridiculous, it is wasteful.

End of story.

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Re: DIFFERENTIALS & TRANSFER CASE

Post by onourway » Wed Feb 06, 2019 9:08 pm

Dontridetheindexdown wrote:
Wed Feb 06, 2019 9:00 pm
For what it's worth:

1. Left and right tires on each axle turn at different speeds. That is why we have a "differential" on each drive axle. If you remove the differential cover, you will see a spider gear (4 helical-cut gears) that allow the left and right wheels to turn at at different speeds, including one side not turning at all, or even turning backwards. Without differential gears, you would not be able to turn a corner on paved roads without scrubbing your tires or skidding. Whether you are speeding down the interstate, or just meandering down a dirt road, the left and right tires on each axle are always turning at different speeds.

2. With all wheel drive, there is a third differential in the transfer case. This allows the front and rear axles to turn at different speeds. Open up the transfer case and look at the third differential if you do not believe me. Old-style 4 wheel drive vehicles did not have a differential in the transfer case. That worked in off-road conditions (sand, snow, or mud). On paved roads, the old-style 4 wheel drive vehicles exhibited serious handling problems, especially at highway speed, and especially on paved roads.

3. All wheel drive vehicles use traction control (automatic braking system (ABS) sensors/actuators to control "slip," and/or limited slip differentials) to moderate the differentiation between left and right tires on both front and rear axles, and also to moderate the differentiation between front and rear axles. The best modern vehicles will "cruise control" at high speed on slick roads, continually modifying the differentiation between tires on the same axle, and between the front and rear axles.

4. There is no reason whatsoever to change all 4 tires when one tire needs replacement. All 4 tires rotate at speeds independent of each other, no matter what type of vehicle you drive. The crown of the road, the amount of debris on the road, and the fact that all vehicles turn left and right, require that each tire turns independently, at its own speed.

5. Feel free to discount my input. I have more than 50 years experience in fleet maintenance, including tactical military and emergency vehicles. Do any of you believe that emergency vehicles get all 6 or all 10 tires replaced when one tire fails?

6. Feel free to do whatever you wish. I know for a fact there is no way whatsoever that you can damage a differential or a transmission by replacing just one tire.

7. Tire shaving is beyond ridiculous, it is wasteful.

End of story.
Fortunately you are certain you know more than the engineers at Subaru.

Dontridetheindexdown
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Re: replacing tires on AWD cars do you have to do all 4

Post by Dontridetheindexdown » Wed Feb 06, 2019 9:20 pm

Yes, as a matter of fact I do.

My wife drives a Subaru Forester, Symmetrical All-wheel Drive.

I have read and mostly memorized the Subaru factory service manuals.

There is no engineer working today at Subaru with as much experience as I have.

In Japan, there are extremely strict age restrictions on both vehicles and employees.

I have a certain respect for Japanese engineering, and for their social system.

However, I have been in this business since 1970, when Subaru was still trying to find their niche in the world.

I stand by my posting.

Feel free to calculate the difference in wheel speed between a brand new tire, and a worn down tire.

Let me know if you can prove any difference in stress on a differential, a transfer case, or a transmission.
Last edited by Dontridetheindexdown on Wed Feb 06, 2019 9:22 pm, edited 1 time in total.

The Wizard
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Re: DIFFERENTIALS & TRANSFER CASE

Post by The Wizard » Wed Feb 06, 2019 9:21 pm

onourway wrote:
Wed Feb 06, 2019 9:08 pm
Dontridetheindexdown wrote:
Wed Feb 06, 2019 9:00 pm
For what it's worth:

1. Left and right tires on each axle turn at different speeds. That is why we have a "differential" on each drive axle. If you remove the differential cover, you will see a spider gear (4 helical-cut gears) that allow the left and right wheels to turn at at different speeds, including one side not turning at all, or even turning backwards. Without differential gears, you would not be able to turn a corner on paved roads without scrubbing your tires or skidding. Whether you are speeding down the interstate, or just meandering down a dirt road, the left and right tires on each axle are always turning at different speeds.

2. With all wheel drive, there is a third differential in the transfer case. This allows the front and rear axles to turn at different speeds. Open up the transfer case and look at the third differential if you do not believe me. Old-style 4 wheel drive vehicles did not have a differential in the transfer case. That worked in off-road conditions (sand, snow, or mud). On paved roads, the old-style 4 wheel drive vehicles exhibited serious handling problems, especially at highway speed, and especially on paved roads.

3. All wheel drive vehicles use traction control (automatic braking system (ABS) sensors/actuators to control "slip," and/or limited slip differentials) to moderate the differentiation between left and right tires on both front and rear axles, and also to moderate the differentiation between front and rear axles. The best modern vehicles will "cruise control" at high speed on slick roads, continually modifying the differentiation between tires on the same axle, and between the front and rear axles.

4. There is no reason whatsoever to change all 4 tires when one tire needs replacement. All 4 tires rotate at speeds independent of each other, no matter what type of vehicle you drive. The crown of the road, the amount of debris on the road, and the fact that all vehicles turn left and right, require that each tire turns independently, at its own speed.

5. Feel free to discount my input. I have more than 50 years experience in fleet maintenance, including tactical military and emergency vehicles. Do any of you believe that emergency vehicles get all 6 or all 10 tires replaced when one tire fails?

6. Feel free to do whatever you wish. I know for a fact there is no way whatsoever that you can damage a differential or a transmission by replacing just one tire.

7. Tire shaving is beyond ridiculous, it is wasteful.

End of story.
Fortunately you are certain you know more than the engineers at Subaru.
Unfortunately, Subaru has designed their transmission to fail if tires aren't close to same diameter.
Clearly they are cheaping out somewhere in differential engineering...
Attempted new signature...

Dontridetheindexdown
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Re: replacing tires on AWD cars do you have to do all 4

Post by Dontridetheindexdown » Wed Feb 06, 2019 9:24 pm

I have not seen any evidence that Subaru transmissions are designed to fail, under any circumstances.

Nor have I seen any evidence that their transfer case or differential is "designed to fail."

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Re: replacing tires on AWD cars do you have to do all 4

Post by The Wizard » Wed Feb 06, 2019 9:32 pm

Dontridetheindexdown wrote:
Wed Feb 06, 2019 9:24 pm
I have not seen any evidence that Subaru transmissions are designed to fail, under any circumstances.

Nor have I seen any evidence that their transfer case or differential is "designed to fail."
We'll have to let Subaru owners enlighten us on this then.

I'm an F-150 owner, with a transmission that has a "4A" selection, in addition to normal 2WD and two 4WD options.
Plus e-locking rear differential for tough off-road pulling.

And no, I'm not replacing all 4 tires if one goes early.
Hopefully all four grow old gracefully...
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The Wizard
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Re: DIFFERENTIALS & TRANSFER CASE

Post by The Wizard » Wed Feb 06, 2019 9:46 pm

Spirit Rider wrote:
Wed Feb 06, 2019 9:40 pm
The Wizard wrote:
Wed Feb 06, 2019 9:21 pm
Unfortunately, Subaru has designed their transmission to fail if tires aren't close to same diameter. Clearly they are cheaping out somewhere in differential engineering...
Does that apply to all the other AWD manufacturers in the the world with similar specifications.
Apparently Subarus have more stringent specs in this area, beyond which they self-distruct?
I'm not planning to have one so you'll have to caucus amongst co-owners to ferret out the experience-based truth...
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Re: replacing tires on AWD cars do you have to do all 4

Post by Spirit Rider » Wed Feb 06, 2019 9:47 pm

The Wizard wrote:
Wed Feb 06, 2019 9:32 pm
I'm an F-150 owner, with a transmission that has a "4A" selection, in addition to normal 2WD and two 4WD options.
Plus e-locking rear differential for tough off-road pulling.

And no, I'm not replacing all 4 tires if one goes early. Hopefully all four grow old gracefully...
I guess you don't trust Ford. From the F-150 owner's manual; "The two front tires or two rear tires should generally be replaced as a pair, except if the vehicle is equipped with four wheel drive. Vehicles equipped with four wheel drive should have all four tires replaced simultaneously."

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Re: replacing tires on AWD cars do you have to do all 4

Post by RudyS » Wed Feb 06, 2019 9:49 pm

Need data please!!!! Any Subaru owners whose 4 wheel drive differential has failed due to tire mismatch? Or any reason, for that matter?

Starfish
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Re: replacing tires on AWD cars do you have to do all 4

Post by Starfish » Wed Feb 06, 2019 9:51 pm

livesoft wrote:
Wed Feb 06, 2019 8:24 pm
Starfish wrote:
Wed Feb 06, 2019 7:52 pm
So how do you match the tire pressure which should affect the effective radius more than 1 mm of rubber?
One uses a tire pressure gauge. If the car is put up on a rack with the wheels hanging, then one can measure the circumferences of all the tires. One can even change the air pressure in the tires and make multiple measurements. Then plot Pressure vs Circumference.

I would guess that a 5 psi difference would not make much difference. Inside the door well of my Subaru, the recommended tire pressures are different by 2 PSI for the front and rear tires. However, the dealership so far has always put the same PSI in all 4 tires and always 2 to 3 PSI lower than what is on the sticker in the door well. Since the car has tire pressure monitoring system, one can see the tire pressures vary depending on ambient temperature, elevation, which side of the car was sitting in strong sunlight, and driving time. The TPMS matches what I measure with a handheld tire pressure gauge.
Pressure gauge (which anyway few people use or care about 1-2psi) is irrelevant because tires lose air at different rates and can have different temperatures.
If you have a bad tire which becomes deflated is gonna break your differential?

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Re: replacing tires on AWD cars do you have to do all 4

Post by livesoft » Wed Feb 06, 2019 9:52 pm

Starfish wrote:
Wed Feb 06, 2019 9:51 pm
If you have a bad tire which becomes deflated is gonna break your differential?
No, it will not. You will get a warning that your tire pressure is low beforehand and will do something about it.
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madpunster
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Re: replacing tires on AWD cars do you have to do all 4

Post by madpunster » Wed Feb 06, 2019 9:54 pm

Our 2005 Forester had 10k on a set of 4 when one of our tires developed a split in the sidewall when traveling (we had 190k on the vehicle). Under warranty, the tire was replaced with the same brand and model. On returning home, we asked our independent Subie mechanic whether we should have changed all four - he answered "No way!". So we kept on going for another 30k until the tires showed some dry rot and we put on a new set. So for what it's worth, in our particular instance, it worked out ok.

Starfish
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Re: replacing tires on AWD cars do you have to do all 4

Post by Starfish » Wed Feb 06, 2019 9:55 pm

The sensor is very imprecise and it is only a recent addition. It can overlook easily 5psi.
What is the difference in effective circumference with 5psi difference?

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Re: replacing tires on AWD cars do you have to do all 4

Post by livesoft » Wed Feb 06, 2019 9:57 pm

My sensors do not overlook a 5 psi change. They are more than sensitive and precise enough. Are they accurate? They are more than accurate enough. Can they over time become less accurate? I don't see why they cannot become less accurate.

As for circumference change, I don't know the answer, but I already posted a way for someone to figure that out.
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jstat
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Re: replacing tires on AWD cars do you have to do all 4

Post by jstat » Wed Feb 06, 2019 9:59 pm

We have had the dealer replace a single tire on two of our Subarus, a 2108 Outback at 4,500 miles, and an STI. In the case of the STI the dealer said it would be fine if they shaved the tire, and after thousands of miles of spirited driving the car is fine. I don't recall if they shaved the Outback tire, but 9,000 highway miles later the car is fine.

If they dealer had said we needed to replace all four tires I would have done it, but this is what they recommended.

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Re: replacing tires on AWD cars do you have to do all 4

Post by Morgan Dollar 1921 » Wed Feb 06, 2019 10:03 pm

onourway wrote:
Wed Feb 06, 2019 7:31 pm
Yes, with your Subaru you should do this. Ask your mechanic to order one tire from Tire Rack shaved to the depth of the remaining three tires. They charge about $15 for this service.
Definitely the right answer, I own one and worked in collision repair for a major dealer chain in Ohio that has two Subaru dealer points and one collision repair center for nine years. A collision often smokes one or two tires, and insurance would almost always charge betterment on tire replacement due to worn tires on the vehicle, and new tread being installed. The Subaru AWD is one of the most advanced AWD systems unless newer technology has been implemented since my retirement in May 2016.

Replace all four tires with NEW tires or pay Tire Rack to shave one or two to the current tread wear depth. It really is a case of pay me now or pay me later, and the AWD unit is more expensive.

Oh, third option trade it in for a new/used vehicle of your choice. :happy

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LiveSimple
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Re: replacing tires on AWD cars do you have to do all 4

Post by LiveSimple » Wed Feb 06, 2019 10:03 pm

It depends, if we have used the tires more and had to replace one, will replace all 4 tires.
Also depends on if we plan to keep the car to ourselves for the next 3+ years.

Do not feel comfortable to add a brand new tire with old tires, may be safety issues or engineering issues later.

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Re: replacing tires on AWD cars do you have to do all 4

Post by The Wizard » Wed Feb 06, 2019 10:05 pm

Spirit Rider wrote:
Wed Feb 06, 2019 9:47 pm
The Wizard wrote:
Wed Feb 06, 2019 9:32 pm
I'm an F-150 owner, with a transmission that has a "4A" selection, in addition to normal 2WD and two 4WD options.
Plus e-locking rear differential for tough off-road pulling.

And no, I'm not replacing all 4 tires if one goes early. Hopefully all four grow old gracefully...
I guess you don't trust Ford. From the F-150 owner's manual; "The two front tires or two rear tires should generally be replaced as a pair, except if the vehicle is equipped with four wheel drive. Vehicles equipped with four wheel drive should have all four tires replaced simultaneously."
I'm hoping to replace all four tires with higher load-rated ones in a few years, checko...
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Retrograde
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Re: replacing tires on AWD cars do you have to do all 4

Post by Retrograde » Wed Feb 06, 2019 10:06 pm

I've never heard this about a differential or transmission failing because one tire was more worn than the others. Given how differentials work, every time your make a turn, your wheels are turning at different speeds. The outer wheel has to travel a further distance and therefore needs to spin faster, otherwise either the inner or outer wheel will slip. The purpose of a differential is to account for this. What would break your differential is if you were stuck in the snow and all the power was going to the wheel that was spinning freely; and you just held down the pedal for a while the diff would heat up and eventually break.

The main reasons places might recommend to replace all four is first making money, second reason is so the tires all wear out at roughly the same time. Some people don't want to deal with needing to replace a single tire every time they take the car in.

The Wizard
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Re: replacing tires on AWD cars do you have to do all 4

Post by The Wizard » Wed Feb 06, 2019 10:07 pm

I found this here statement on The Internet:

Four-wheel drive and all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail.

So there we have it!
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Retrograde
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Re: replacing tires on AWD cars do you have to do all 4

Post by Retrograde » Wed Feb 06, 2019 10:22 pm

I've read some of the various posts after reading the post and maybe Subarus have this issue. But it sounds truly bizarre that the transmission and differentials wouldn't be able to handle the difference in tire tread depth. Makes me glad I bought a Ford instead of a WRX.

Morgan Dollar 1921
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Re: replacing tires on AWD cars do you have to do all 4

Post by Morgan Dollar 1921 » Wed Feb 06, 2019 10:30 pm

Here are some tests from youtube:

Subaru AWD Comparison Test

Subaru AWD Driving Traction vs. The Competition 2015 Version

onourway
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Re: replacing tires on AWD cars do you have to do all 4

Post by onourway » Wed Feb 06, 2019 10:32 pm

Retrograde wrote:
Wed Feb 06, 2019 10:06 pm
I've never heard this about a differential or transmission failing because one tire was more worn than the others. Given how differentials work, every time your make a turn, your wheels are turning at different speeds. The outer wheel has to travel a further distance and therefore needs to spin faster, otherwise either the inner or outer wheel will slip. The purpose of a differential is to account for this. What would break your differential is if you were stuck in the snow and all the power was going to the wheel that was spinning freely; and you just held down the pedal for a while the diff would heat up and eventually break.

The main reasons places might recommend to replace all four is first making money, second reason is so the tires all wear out at roughly the same time. Some people don't want to deal with needing to replace a single tire every time they take the car in.
The difference is that you are only turning for a brief period of time. When tires are of differing diameters, the differential is forced to work constantly mile after mile, for tens of thousands of miles. It is not designed for this.

(I see the same answer was already posted!)

mhalley
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Re: replacing tires on AWD cars do you have to do all 4

Post by mhalley » Wed Feb 06, 2019 10:32 pm

One thing about all these warnings is none specifies how long it would take for the different tire circumference to cause differential failure, I imagine it would take a fair amount of miles. But is it 10k, 20k, 50k? Subaru usually does pretty well on the CR reliability surveys, does that mean most people are replacing all 4 tires, or that it takes a large number of miles to differential failure?
Last edited by mhalley on Wed Feb 06, 2019 10:34 pm, edited 1 time in total.

onourway
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Re: replacing tires on AWD cars do you have to do all 4

Post by onourway » Wed Feb 06, 2019 10:32 pm

Retrograde wrote:
Wed Feb 06, 2019 10:22 pm
I've read some of the various posts after reading the post and maybe Subarus have this issue. But it sounds truly bizarre that the transmission and differentials wouldn't be able to handle the difference in tire tread depth. Makes me glad I bought a Ford instead of a WRX.
As noted previously, your Ford owner's manual contains the same warning.

Teague
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Re: DIFFERENTIALS & TRANSFER CASE

Post by Teague » Wed Feb 06, 2019 10:33 pm

Dontridetheindexdown wrote:
Wed Feb 06, 2019 9:00 pm
For what it's worth:


3. All wheel drive vehicles use traction control (automatic braking system (ABS) sensors/actuators to control "slip," and/or limited slip differentials) to moderate the differentiation between left and right tires on both front and rear axles, and also to moderate the differentiation between front and rear axles. The best modern vehicles will "cruise control" at high speed on slick roads, continually modifying the differentiation between tires on the same axle, and between the front and rear axles.
There is at least one car where the ABS/traction control/stability control will throw codes if the tire circumference is a bit different. I own such a car (2003 Explorer.) I know this because, after replacing a single tire, that system frequently reported a problem and would shut itself down. The car was under warranty so I took it to the dealer, who told me the problem was the new tire. I was incredulous, figuring the lazy dealership just wanted to be done with me and made this up. The service adviser said no, this was in fact a common issue. I still didn't believe it.

Eventually, I got tired of the codes and dash warning lights, and installed the three other new tires. I was not expecting any result other than being poorer. However, the problem was solved.
Semper Augustus

Nate79
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Re: replacing tires on AWD cars do you have to do all 4

Post by Nate79 » Wed Feb 06, 2019 10:39 pm

If you watch the Subaru forums there have been a few anecdotes of diff damage due to tire tread differences. Most say the spec requirement is probably overkill but there comes a point where damage will occur. I recall a few years ago that a large shop owner mentioned that they have seen a number of awd cars over the years have diff damage due to tires.

It is not a made up requirement.

Morgan Dollar 1921
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Re: replacing tires on AWD cars do you have to do all 4

Post by Morgan Dollar 1921 » Wed Feb 06, 2019 10:49 pm

I have been "on the tile in the dealer's office for telling a customer in the service dept that you need to trust at least three people in your life, your spouse, your doctor, and the shop that does your auto service.He said however true it may be some customers simply don't have my sense of humor. I stated "Dave it has nothing to do with humor they are driving down the road traveling over a 80 feet per second and my kids are on a school bus headed home.

Find someone you trust with your car and stick with them.

"You can trust your car to the man who wears the star, the big bright Texaco star" Check that jingle on youtube, and you will see him installing tire chains.

I don't miss those days.

wootwoot
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Re: replacing tires on AWD cars do you have to do all 4

Post by wootwoot » Wed Feb 06, 2019 10:52 pm

Many times you can change 2 tires on the same axle rather than replacing all 4 but it depends on the tolerances that Subaru provides for your vehicle.

Teague
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Re: replacing tires on AWD cars do you have to do all 4

Post by Teague » Thu Feb 07, 2019 12:26 am

Morgan Dollar 1921 wrote:
Wed Feb 06, 2019 10:49 pm

Find someone you trust with your car and stick with them.

"You can trust your car to the man who wears the star, the big bright Texaco star" Check that jingle on youtube, and you will see him installing tire chains.
Unfortunately, the customer only came in for an oil change. :wink:
Semper Augustus

aqan
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Re: replacing tires on AWD cars do you have to do all 4

Post by aqan » Thu Feb 07, 2019 12:38 am

arf30 wrote:
Wed Feb 06, 2019 7:31 pm
You're supposed to replace all 4 to avoid wear on the differentials. You can also buy one new tire and have it "shaved" down to the correct diameter but I've never done this.
My local flat repair shop sold me a used tire with similar tread as my other 3 for $15 + mounting. I even got the same brand Yokohama. OP try this or even craigslist/fb marketplace as well.

michaeljc70
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Re: replacing tires on AWD cars do you have to do all 4

Post by michaeljc70 » Thu Feb 07, 2019 12:49 am

A friend that had a Mercedes AWD was told they needed to replace all the tires as the specifications were so tight. It wasn't even a tire that couldn't be repaired. They were told the weight of the repair would be an issue. I have an AWD Mazda and if a tire goes out, I would not replace all 4 tires. If I had to due to issues, I would probably never buy an AWD vehicle again as it seems like an absurd waste.

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