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

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28fe6
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Re: replacing tires on AWD cars do you have to do all 4

Post by 28fe6 » Thu Feb 07, 2019 1:16 am

onourway wrote:
Wed Feb 06, 2019 10:32 pm
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!)

No, you are almost always turning when you are driving. How long could you drive without turning the steering wheel at all? Hardly at all, so the differential is already "forced to work constantly mile after mile", even when tires are identical. Even straight sections highway require gentle steering input, and that's just as much differential action as any difference in tire wear. Unless you think that turning your steering wheel is something your car was "not designed for" then there's no way differences in tire wear could be significant. It is s good idea to use the same model of tires and to fill them to the recommended PSI, but don't worry about tire wear differences. It's just a ploy to sell tires.

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

Post by Starfish » Thu Feb 07, 2019 1:20 am

livesoft wrote:
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.
You know that actually you can reset the sensors yourself right? You can tell them what is the good pressure and they believe you! If you reset them with unequal pressure in tires, there is no precision.
I have more questions: if I drive a Subaru in a turn does it break? Because I drive thousands of miles a year turning one way or another and the wheels always have some difference in speed due to turning.
So many questions, so few answers :D

inbox788
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Re: replacing tires on AWD cars do you have to do all 4

Post by inbox788 » Thu Feb 07, 2019 3:05 am

28fe6 wrote:
Thu Feb 07, 2019 1:16 am
No, you are almost always turning when you are driving. How long could you drive without turning the steering wheel at all? Hardly at all, so the differential is already "forced to work constantly mile after mile", even when tires are identical. Even straight sections highway require gentle steering input, and that's just as much differential action as any difference in tire wear. Unless you think that turning your steering wheel is something your car was "not designed for" then there's no way differences in tire wear could be significant.
I go out of my way to make right turns instead of left. Not only are right turns safer than left turns, but it's easier running errands following right hand hand rule, so I plan my routes with that in mind. Getting in and out of a gas station making right turns is simpler, so I visit them when they're on the right side. I probably make 90% right turns and only 10% left. I always wondered if the right side of my tires would wear out more, but after thousands of miles, I've yet to notice a difference. They're not 4WD, but I don't think it would make much of a difference. Also, the right turns are usually tighter turns, so you'd expect it to put even more wear than making wider left turns. Come to think of it, maybe it's the slightly longer path on the left side of the tire when making right turns that gets more wear, so I may have been looking at it wrong. :confused

The case for almost never turning left while driving - Washington Post
https://www.washingtonpost.com/news/inn ... e-driving/ [sorry, maybe paywall]

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

Post by The Wizard » Thu Feb 07, 2019 5:57 am

inbox788 wrote:
Thu Feb 07, 2019 3:05 am
...The case for almost never turning left while driving - Washington Post
https://www.washingtonpost.com/news/inn ... e-driving/ [sorry, maybe paywall]
This is great, but has little to do with AWD cars with mismatched tires...
Attempted new signature...

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

Post by Nicolas » Thu Feb 07, 2019 8:34 am

aqan wrote:
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.
According to this Edmunds.com article old tires may be dangerous to drive on due to rubber aging. If you buy a used tire how do you know its age? https://www.edmunds.com/car-maintenance ... tires.html

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

Post by tibbitts » Thu Feb 07, 2019 8:39 am

Nicolas wrote:
Thu Feb 07, 2019 8:34 am
aqan wrote:
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.
According to this Edmunds.com article old tires may be dangerous to drive on due to rubber aging. If you buy a used tire how do you know its age? https://www.edmunds.com/car-maintenance ... tires.html
When you select the tire you can check the date code stamped on the sidewall. Once even a new (spare) tire I bought online was already half through its theoretical life, but I kept it instead of return it. In retrospect I should have asked when ordering it.

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

Post by unstartable » Thu Feb 07, 2019 8:52 am

For OP, you should following the instructions in your owners manual.

I think the problem has the most to do with how the manufacture has implemented some type of limited slip system into the differentials. An open differential will not overheat or have other problems if the tires are different sizes. The central differentials in AWD vehicles are not a simple open differential; all have some type of system to send some amount of torque to the axle with more traction. These systems vary widely, and may be more or less sensitive to differences in wheel size.

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

Post by jharkin » Thu Feb 07, 2019 8:53 am

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.
Yes and no - lets not scare people unnecessarily.

An AWD vehicle has 3 differentials - one on each axle, and one in the center. If they Torsen style limited slip, having mismatched diameters may cause them to run hotter and may someday, eventually, possibly, cause more wear. If they are open differentials, having a minor difference in rolling diameter likely wont make a bit of difference.

Some system that use open differentials with electronic traction control to mimic limited slip, or systems that do active torque vectoring with electronic clutch packs (like Honda/Acura) - these may get confused if the diameter difference is big enough and cause traction control to kick in when its not needed.

Traditional "4x4" shiftable 4WD (think pickup, Jeep, Blazer, 4Runner, etc) - the part time systems with locking center transfer cases - will not like having mismatches wheels front to back and one axle will drag/skid. Its not likely to actually break the transfer case however but it will put a lot of stress on everything and wear out the tires faster. Note that these systems drive awful in locked 4x4 mode on dry pavement anyway so you shouldn't be in 4Hi/4Lo unless its pretty slick to begin with.

Bottom line - YES I do all 4 tires if the other tires have a lot of miles.. But I have replaced just one when the others where near new (under 10k miles) on a Honda torque vectoring AWD and got away with it no issues.
Last edited by jharkin on Thu Feb 07, 2019 9:00 am, edited 2 times in total.

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

Post by jabberwockOG » Thu Feb 07, 2019 8:53 am

Tires date codes are useful and should be checked regularly but serve only as a guide. But be aware that tire ageing varies a lot based on the environment they operate in. UV rays, corrosive and/or oxidizing air exposure, and high temperatures age tires. Tires mounted on a car that lives outside in the hot sun in Texas likely age at 3x the rate of same tires on a garage kept car in Washington state. A spare in the trunk ages much more slowly than one mounted on the car. But I personally would not run a tire even if it looked great that is more than 10-12 years old.

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

Post by jharkin » Thu Feb 07, 2019 8:58 am

michaeljc70 wrote:
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.
Just go to show you that even the people that work a dealership dont actually have a clue. :oops:

Tire wight is meaningless in this discussion. Only the rolling diameter matters. And as have been noted multiple times the small difference in rolling diameter of a tire with a few thousand miles more use is less than the difference the differential is seeing all day long from turning, bumps in the pavement, and so on.

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

Post by Nicolas » Thu Feb 07, 2019 9:05 am

tibbitts wrote:
Thu Feb 07, 2019 8:39 am
Nicolas wrote:
Thu Feb 07, 2019 8:34 am
aqan wrote:
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.
According to this Edmunds.com article old tires may be dangerous to drive on due to rubber aging. If you buy a used tire how do you know its age? https://www.edmunds.com/car-maintenance ... tires.html
When you select the tire you can check the date code stamped on the sidewall. Once even a new (spare) tire I bought online was already half through its theoretical life, but I kept it instead of return it. In retrospect I should have asked when ordering it.
Okay, three of my tires are nearly seven years old, I checked the DOT date codes, as well as the receipt for when I bought them (new) with the car’s mileage at the time. They have 55,000 miles on them and still have some tread life left. The fourth tire I bought new in November last year when the original failed and it was built in March so it was already 1/2 year old when I bought it, not too bad I suppose.

I’ve been agonizing over this decision since first reading this thread, but due to the tires’ age my decision has been easier.

There is a significant difference in tread wear between the newest tire and the older ones as might be imagined. Due to these tires having greatly different tread depth and since these tires are mounted on my AWD 2006 Subaru Outback I’ve decided to bite the bullet and replace the three older tires now, and I’m not keeping them for future use due to their advanced age.

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

Post by Spirit Rider » Thu Feb 07, 2019 9:31 am

Yes, saving the best tires on replacement really only works well if you are replacing after five years or less.

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

Post by lthenderson » Thu Feb 07, 2019 9:50 am

This whole discussion makes me happy that I own a AWD Toyota instead of a AWD Subaru. Toyota shuts down their AWD system under normal driving conditions so the vehicle is essentially front wheel drive only. This allows for replacing pairs of tires instead of all four so that you don't strain your differential by having it work all the time with uneven tires. It also gives better fuel economy. Then when AWD is needed, it kicks in and applies torque to the appropriate tires.

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

Post by El Greco » Thu Feb 07, 2019 9:58 am

RudyS wrote:
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?
My 1998 Subaru Outback Transfer case differential began to fail at 75,000 miles. I don't know why. It had 2 complete sets of new tires for the entire time I owned it.

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

Post by The Wizard » Thu Feb 07, 2019 10:31 am

lthenderson wrote:
Thu Feb 07, 2019 9:50 am
This whole discussion makes me happy that I own a AWD Toyota instead of a AWD Subaru. Toyota shuts down their AWD system under normal driving conditions so the vehicle is essentially front wheel drive only. This allows for replacing pairs of tires instead of all four so that you don't strain your differential by having it work all the time with uneven tires. It also gives better fuel economy. Then when AWD is needed, it kicks in and applies torque to the appropriate tires.
I was almost gunna ask about this: do AWD vehicles have a switch to go between 2WD and AWD?

As I mentioned earlier, my F-150 has such a switch and I leave it in 2WD most of the time on paved roads.
But with snow or rain, I'll switch to 4A sometimes, especially at stop signs with no load in the bed. 4A is 4WD "Automatic" where torque is sent to front wheels as needed somehow.
There's even a display I can call up that shows dynamic torque distribution.

Anyhow, better to keep drive train simple when complexity isn't needed...
Attempted new signature...

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

Post by Starfish » Thu Feb 07, 2019 12:39 pm

Nicolas wrote:
Thu Feb 07, 2019 9:05 am
tibbitts wrote:
Thu Feb 07, 2019 8:39 am
Nicolas wrote:
Thu Feb 07, 2019 8:34 am
aqan wrote:
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.
According to this Edmunds.com article old tires may be dangerous to drive on due to rubber aging. If you buy a used tire how do you know its age? https://www.edmunds.com/car-maintenance ... tires.html
When you select the tire you can check the date code stamped on the sidewall. Once even a new (spare) tire I bought online was already half through its theoretical life, but I kept it instead of return it. In retrospect I should have asked when ordering it.
Okay, three of my tires are nearly seven years old, I checked the DOT date codes, as well as the receipt for when I bought them (new) with the car’s mileage at the time. They have 55,000 miles on them and still have some tread life left. The fourth tire I bought new in November last year when the original failed and it was built in March so it was already 1/2 year old when I bought it, not too bad I suppose.

I’ve been agonizing over this decision since first reading this thread, but due to the tires’ age my decision has been easier.

There is a significant difference in tread wear between the newest tire and the older ones as might be imagined. Due to these tires having greatly different tread depth and since these tires are mounted on my AWD 2006 Subaru Outback I’ve decided to bite the bullet and replace the three older tires now, and I’m not keeping them for future use due to their advanced age.

It is nothing to agonize about, just change all of them. I would not drive around with 55000 miles 7 year old tires. They were crappy to begin with (decent tires don't last that much) and the exposure to UV alone degrades a tire in 5 years even if you don't drive it. So even if they had 0 miles it was questionable but at 55k miles? no way.
As I mentioned, I drive over a hill every day. Every time it rains it's full of accidents. Probably is 7y/55k miles tires people.

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

Post by lthenderson » Thu Feb 07, 2019 1:08 pm

The Wizard wrote:
Thu Feb 07, 2019 10:31 am
lthenderson wrote:
Thu Feb 07, 2019 9:50 am
This whole discussion makes me happy that I own a AWD Toyota instead of a AWD Subaru. Toyota shuts down their AWD system under normal driving conditions so the vehicle is essentially front wheel drive only. This allows for replacing pairs of tires instead of all four so that you don't strain your differential by having it work all the time with uneven tires. It also gives better fuel economy. Then when AWD is needed, it kicks in and applies torque to the appropriate tires.
I was almost gunna ask about this: do AWD vehicles have a switch to go between 2WD and AWD?
The two AWD Toyotas I have do not have a switch to turn off AWD. The vehicle controls the AWD aspect. There is a switch to control traction control. My understanding of the Subarus is that they don't have a switch either and unlike the Toyotas, they truly are AWD all the time.

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

Post by Nicolas » Thu Feb 07, 2019 1:21 pm

Starfish wrote:
Thu Feb 07, 2019 12:39 pm
Nicolas wrote:
Thu Feb 07, 2019 9:05 am
Okay, three of my tires are nearly seven years old, I checked the DOT date codes, as well as the receipt for when I bought them (new) with the car’s mileage at the time. They have 55,000 miles on them and still have some tread life left. The fourth tire I bought new in November last year when the original failed and it was built in March so it was already 1/2 year old when I bought it, not too bad I suppose.

I’ve been agonizing over this decision since first reading this thread, but due to the tires’ age my decision has been easier.

There is a significant difference in tread wear between the newest tire and the older ones as might be imagined. Due to these tires having greatly different tread depth and since these tires are mounted on my AWD 2006 Subaru Outback I’ve decided to bite the bullet and replace the three older tires now, and I’m not keeping them for future use due to their advanced age.

It is nothing to agonize about, just change all of them. I would not drive around with 55000 miles 7 year old tires. They were crappy to begin with (decent tires don't last that much) and the exposure to UV alone degrades a tire in 5 years even if you don't drive it. So even if they had 0 miles it was questionable but at 55k miles? no way.
As I mentioned, I drive over a hill every day. Every time it rains it's full of accidents. Probably is 7y/55k miles tires people.
I agree with you now. I was agonizing but no longer. In fact my mechanic is installing my new tires as I write this. By coincidence I already had an appointment scheduled today for a different repair so I just added the new tires.

I will be following this advice from now on as well. I’ve always tried to squeeze all the value out of things before replacement. All my life I’ve tried to be frugal and this extended to car maintenance.
Last edited by Nicolas on Fri Feb 08, 2019 8:23 am, edited 1 time in total.

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

Post by whodidntante » Thu Feb 07, 2019 1:47 pm

Follow the manufacturer instructions. There is a fun and informative book that came with your car. It contains information that your car manufacturer thought you should know. It can be used to avoid contentious discussions online about maintenance including engine oil. If someone pawned the book, then download a copy. Actually that is probably better because then you can search the book for "tire."

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

Post by inbox788 » Thu Feb 07, 2019 1:50 pm

The Wizard wrote:
Thu Feb 07, 2019 5:57 am
This is great, but has little to do with AWD cars with mismatched tires...
Not directly, but the discussion was over steering and turning (which under normal driving don't seem to affect tire wear, either between tires or even side to side, even when done asymmetrically - effect must be small or some countering effects may be involved). Tires experience different forces and wear based on what is done to them and where they're located. RWD cars experience less uneven wear than FWD cars because some of the wear offset each other (wear from braking vs accelerating and which axle is impacted more).

If the right front tire gets the most wear, one simple solution is to put the new tire or the tire with the most thread left on the right front position. Similarly, the least thread tire goes in the rear left position.

Starting with even tires and rotating frequently is the best way to maintain even wear, but when things get uneven, it helps to rotate the tires into positions that tend to even out wear rather than exaggerate the effects, considering all the factors that may or may not play a significant role.

https://www.quora.com/Do-the-tires-on-o ... -the-other
In places like the US where they drive on the right, the problem is reversed. The front right takes the brunt of daily driving and wears faster.
http://toyotires.com.au/toyo-blog/540-w ... ear-faster

FWIW, if I only replace one tire, I try to put in the front right position, and try to leave it there for the rest of the life of the tires. No point of rotating it out of position if it's got the most thread and biggest diameter that's wearing down faster there. Any potential problems show up faster and over time diminish.

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

Post by jstat » Thu Feb 07, 2019 2:06 pm

I was at the Subaru dealer this morning, and asked the service guy when it was okay to replace just one tire. He said that Subaru requires all tires to have tread depth within 1/16 inch of each other.

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

Post by The Wizard » Thu Feb 07, 2019 2:49 pm

jstat wrote:
Thu Feb 07, 2019 2:06 pm
I was at the Subaru dealer this morning, and asked the service guy when it was okay to replace just one tire. He said that Subaru requires all tires to have tread depth within 1/16 inch of each other.
The party line, checko...
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Re: replacing tires on AWD cars do you have to do all 4

Post by 3feetpete » Thu Feb 07, 2019 5:48 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.
Thank you onourway for your suggestion. I canclelled my order for four tires and ordered one from Tirerack shaved to match my other tires. You saved me almost $600. I owe you a beer! BTW the charge now is $30 for shaving but still worth it.

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

Post by rb6p » Fri Feb 08, 2019 1:04 pm

I went through this for an all wheel drive bmw and arrived at the same conclusion: just buy 1 tire and shave it down.

Tire rack is the only large seller that offered this when I dealt with this last year. They're a great option because they are typically also the cheapest. The tire arrived at the exact tread depth requested. No problems whatsoever. Glad I didn't follow others' advice to replace the other 1 (or 3 tires) unnecessarily.

In the NYC region, I get a slow leak from a nail on the road once every two yrs or so. Not uncommon for it to be un-patch-able. It'd be ridiculous to change 4 tires each time this happened.

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

Post by meowcat » Fri Feb 08, 2019 2:39 pm

Hopefully, I can be of some help. I've worked for an authorized Subaru dealer in the parts and service departments for the past 10 years. Yes, you are supposed to replace any tire that is not within 2/32 tolerance of each other. The reason is what has been mentioned above. Yes, new tires should be shaved to match current tires' depth.
Okay, that's what you're supposed to do. With that, I will tell you, the number of Subaru's that come through our shop with mis-matched tires is staggering, and in the ten years I've been here, we have never, ever had an issue or had to replace any of the 3 differentials because of a tire mis-match. Ever.
More people should learn to tell their dollars where to go instead of asking them where they went. | -Roger Babson

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

Post by inbox788 » Fri Feb 08, 2019 3:41 pm

3feetpete wrote:
Thu Feb 07, 2019 5:48 pm
Thank you onourway for your suggestion. I canclelled my order for four tires and ordered one from Tirerack shaved to match my other tires. You saved me almost $600. I owe you a beer! BTW the charge now is $30 for shaving but still worth it.
Shaving tires is one option that sometimes makes sense, but you're not spending $30 to save $600. The savings are quite a bit less. You're still spending $150 for a tire vs. $600 for 4. Also, how much thread do you new tires have? 12/32? 10/32? 8/32? And do you wear it down to 2/32? or 4/32?

What is your current tread left on the 3 remaining tires? And how many miles since you installed them? What is the mismatch?

I'm not saying don't shave them if it makes sense, but if the tires are nearly new with little mismatch, don't bother. If the tires are almost worn out, might as well take the opportunity to replace all of them.

If the tires are half worn out, you save 3 half worn out tires by shaving off 1/2 wear on a new tire, so you're saving about $75*3, but you're paying $30 to do it, and you're shaving off $75 off the new tire. And you're only getting the latter half, and wasting the better half.

Another way to think about this if a new set of tires gets you 50k miles, you might only get 25k miles remaining miles. So roughly it might make sense for me to do this if the tires have more than 25% but less than 50% wear.

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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 » Sat Feb 09, 2019 4:37 pm

inbox788 wrote:
Fri Feb 08, 2019 3:41 pm
3feetpete wrote:
Thu Feb 07, 2019 5:48 pm
Thank you onourway for your suggestion. I canclelled my order for four tires and ordered one from Tirerack shaved to match my other tires. You saved me almost $600. I owe you a beer! BTW the charge now is $30 for shaving but still worth it.
Shaving tires is one option that sometimes makes sense, but you're not spending $30 to save $600. The savings are quite a bit less. You're still spending $150 for a tire vs. $600 for 4. Also, how much thread do you new tires have? 12/32? 10/32? 8/32? And do you wear it down to 2/32? or 4/32?

What is your current tread left on the 3 remaining tires? And how many miles since you installed them? What is the mismatch?

I'm not saying don't shave them if it makes sense, but if the tires are nearly new with little mismatch, don't bother. If the tires are almost worn out, might as well take the opportunity to replace all of them.

If the tires are half worn out, you save 3 half worn out tires by shaving off 1/2 wear on a new tire, so you're saving about $75*3, but you're paying $30 to do it, and you're shaving off $75 off the new tire. And you're only getting the latter half, and wasting the better half.

Another way to think about this if a new set of tires gets you 50k miles, you might only get 25k miles remaining miles. So roughly it might make sense for me to do this if the tires have more than 25% but less than 50% wear.
Yes I thought of the above however there is also a good chance that it will save me the cost of buying a new set of tires if I sell the car first.

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

Post by whomever » Sun Feb 10, 2019 7:17 am

I'm trying to guesstimate how often we've had to replace a single tire over the years, because of a sidewall puncture or whatever. I'd guess every 7 years or so. If that means an extra $450 to replace three otherwise good tires each time, that's a nontrivial increase in the overall ownership cost over 20 years. I'll keep that in mind the next time I'm shopping for a vehicle.
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."
Some of the upper end F150 models have some flavor of full time 4WD - I think that's the '4A' setting mentioned upthread. I don't have that, and haven't looked at how it works. I have a base F150 with three settings: 2WD, 4H, and 4L (plus the ability to lock the rear differential). I'm not sure whether tire size matters in 4L or 4H (or if the rear diff is locked) - in those modes you're always depending on the tire slipping for even a mild turn; you just don't use those modes, ever, unless the road surface is slippery enough to let the tires slip.

In 2WD mode, which is where you are 99.99% of the time, my reading of the owner's manual is that it is a classic old fashioned 2WD. The front wheels just free wheel, and the rear diff is a classic, not-even-limited-slip differential. That's a very different system than a fulltime AWD. I'm finding it hard to understand why slight tire size differences would be a problem with that.

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

Post by rhornback » Sun Feb 10, 2019 7:47 am

jharkin wrote:
Thu Feb 07, 2019 8:53 am
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.
Yes and no - lets not scare people unnecessarily.

An AWD vehicle has 3 differentials - one on each axle, and one in the center. If they Torsen style limited slip, having mismatched diameters may cause them to run hotter and may someday, eventually, possibly, cause more wear. If they are open differentials, having a minor difference in rolling diameter likely wont make a bit of difference.

Some system that use open differentials with electronic traction control to mimic limited slip, or systems that do active torque vectoring with electronic clutch packs (like Honda/Acura) - these may get confused if the diameter difference is big enough and cause traction control to kick in when its not needed.

Traditional "4x4" shiftable 4WD (think pickup, Jeep, Blazer, 4Runner, etc) - the part time systems with locking center transfer cases - will not like having mismatches wheels front to back and one axle will drag/skid. Its not likely to actually break the transfer case however but it will put a lot of stress on everything and wear out the tires faster. Note that these systems drive awful in locked 4x4 mode on dry pavement anyway so you shouldn't be in 4Hi/4Lo unless its pretty slick to begin with.

Bottom line - YES I do all 4 tires if the other tires have a lot of miles.. But I have replaced just one when the others where near new (under 10k miles) on a Honda torque vectoring AWD and got away with it no issues.
This is a topic that frustrates me because not only is there cost involved but there is also an environmental impact.

When I change tires I always only change two at a time because generally the front tires wear at different rates than the back. Yes I know this is why you are suppose to rotate them but really I do not want to spend my life at the tire store.

My wife has a Honda Pilot with 115,000 miles on it. I only change two tires at a time. I also do not match tire brand (tread) like you are suppose to.

When I buy tires there is always a discount on buying 4 tires. It is pretty common to see buy 3 and get the 4th free. Or a similar discount. But again the tires do not wear evenly so why would I do this.

For this reason I always get my tires at WalMart because they have everyday low prices on tires and do not play games with buy 3 get 1 free.

I could be the fool but so far so good with our Honda Pilot

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

Post by rhornback » Sun Feb 10, 2019 7:53 am

inbox788 wrote:
Fri Feb 08, 2019 3:41 pm
3feetpete wrote:
Thu Feb 07, 2019 5:48 pm
Thank you onourway for your suggestion. I canclelled my order for four tires and ordered one from Tirerack shaved to match my other tires. You saved me almost $600. I owe you a beer! BTW the charge now is $30 for shaving but still worth it.
Shaving tires is one option that sometimes makes sense, but you're not spending $30 to save $600. The savings are quite a bit less. You're still spending $150 for a tire vs. $600 for 4. Also, how much thread do you new tires have? 12/32? 10/32? 8/32? And do you wear it down to 2/32? or 4/32?

What is your current tread left on the 3 remaining tires? And how many miles since you installed them? What is the mismatch?

I'm not saying don't shave them if it makes sense, but if the tires are nearly new with little mismatch, don't bother. If the tires are almost worn out, might as well take the opportunity to replace all of them.

If the tires are half worn out, you save 3 half worn out tires by shaving off 1/2 wear on a new tire, so you're saving about $75*3, but you're paying $30 to do it, and you're shaving off $75 off the new tire. And you're only getting the latter half, and wasting the better half.

Another way to think about this if a new set of tires gets you 50k miles, you might only get 25k miles remaining miles. So roughly it might make sense for me to do this if the tires have more than 25% but less than 50% wear.
I have never shaved tires but I have bought the cheapest tire with the shortest tread warranty to try and compensate. So generally I guy 50 - 60,000 mile tires. If I need to replace a tire due to a sidewall leak and the other tire is a few years old I will put a 40,000 mile tire for the replacement. This has worked for me though admittedly I have never done it on an AWD.

Frankly this experience with my AWD Honda Pilot, and the knowledge that I will never take my vehicles far off road, and the cost of tires on a Honda Pilot vs my Toyota Minivan will make me reconsider AWD on my next vehicle.

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

Post by LadyGeek » Sun Feb 10, 2019 9:27 am

I removed an off-topic post and reply. As a reminder, see: General Etiquette
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Re: replacing tires on AWD cars do you have to do all 4

Post by Big Fish » Sun Feb 10, 2019 10:15 am

Subaru differentials are said to fail often (in terms of random car repairs, probably <10%) starting at about 150k miles. This being from a scouring of the Subaru forums I did a while back when considering purchasing a high-mileage beater.

With that anecdotal evidence mind, it seems possible that Subaru differentials are more fragile than those in other 'highly reliable' drivetrains. I've never heard of a differential failing on a land cruiser or an older full time 4wd 4runner.. but all anecdotal, doesn't really mean anything.

Best to follow the owner's manual.. or make the conscious decision not to (old beater probably doesn't need $600 worth of tires because three have 3/4 tread).

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

Post by F150HD » Sun Feb 10, 2019 10:39 am

3feetpete wrote:
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.
Interesting thread. I see the sidewall comment, but can they put an internal patch in this tire then put a tube in it? so you can run it to the end of its service life. then replace all 4.

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

Post by The Wizard » Sun Feb 10, 2019 11:06 am

whomever wrote:
Sun Feb 10, 2019 7:17 am

...Some of the upper end F150 models have some flavor of full time 4WD - I think that's the '4A' setting mentioned upthread. I don't have that, and haven't looked at how it works. I have a base F150 with three settings: 2WD, 4H, and 4L (plus the ability to lock the rear differential). I'm not sure whether tire size matters in 4L or 4H (or if the rear diff is locked) - in those modes you're always depending on the tire slipping for even a mild turn; you just don't use those modes, ever, unless the road surface is slippery enough to let the tires slip.

In 2WD mode, which is where you are 99.99% of the time, my reading of the owner's manual is that it is a classic old fashioned 2WD. The front wheels just free wheel, and the rear diff is a classic, not-even-limited-slip differential. That's a very different system than a fulltime AWD. I'm finding it hard to understand why slight tire size differences would be a problem with that.
That's exactly my reading as well.
Three years in on my 2016 F-150, I'm still on the original four tires.
If one blows out, I'll just get a single OE Goodyear Wrangler to replace it, without shaving it down.
99% of the time in 2WD, checko.
I did this on my previous pick-up, no problem.

Braking and traction performance will be a bit different on the corner with new rubber, so if you're pushing limits or racing the truck, this won't be the best way to go.
But I don't race my truck that much. :)

As mentioned upthread, I plan to upgrade to higher load-rated tires when the time comes and plan to do all four at once then.
Totally different tires (tread and sidewall character) you should do as a complete set...
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