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Discussion Starter #1 (Edited)
I'm just wondering if this still holds true for the RS's "unique-ish" AWD system.

From TireRack:
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.
But we don't have a diff or viscous coupling. If the rear wheels are already over-driven via gearing I would think that tire tread level would be a minuscule factor compared to the gearing.

Continuing this train of thought, would it even matter if there was a mismatch side to side? We have an open diff up front (which wouldn't care?) and we have the clutch packs in back. The clutch packs by design are slipping all the time. It would seem to me that it wouldn't matter on the RS.
 

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The difference between a new tire and a bald tire is usually 10/32nds or less. I think mismatched brands, different tread patterns, treadwear, and traction ratings would cause more damage than a few 32nds of tread depth.
 

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Discussion Starter #3
The difference between a new tire and a bald tire is usually 10/32nds or less. I think mismatched brands, different tread patterns, treadwear, and traction ratings would cause more damage than a few 32nds of tread depth.
While I agree with you, the common advice given is that if you have to replace 1 tire on an AWD vehicle you have to replace all of them.
 

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Just as a frame of reference, I've worked for Nissan for over 10 years. They have very touchy transfer cases and diffs, so I'm pretty well versed on the ins and outs of what kills parts in traditional awd systems. Even so, I can't speak to the system in the RS because of the significant differences, but i can't see it being more fragile than the nissan systems. And I've never given the advice of changing 4 because of 1. I do however, "recommend" replacing 2 on the same axle when it does happen and they are more than 50% worn.
 

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Discussion Starter #5
Just as a frame of reference, I've worked for Nissan for over 10 years. They have very touchy transfer cases and diffs, so I'm pretty well versed on the ins and outs of what kills parts in traditional awd systems. Even so, I can't speak to the system in the RS because of the significant differences, but i can't see it being more fragile than the nissan systems. And I've never given the advice of changing 4 because of 1. I do however, "recommend" replacing 2 on the same axle when it does happen and they are more than 50% worn.
I'm not saying you're wrong. I'm just saying that I know Discount Tire will tell you to replace all 4 as well as TireRack. The majority of people advise to replace all 4 (or shave a new tire down to the old tires level).

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

https://www.pepboys.com/tires/treadsmart/replacing_tires_on_allwheel_drive_vehicles/

Tire Replacement On All Wheel Drive Vehicles

http://triblive.com/business/headlines/3711495-74/tire-mile-tires#axzz3tDH81bx8
 

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From what I was able to find on the internet regarding my Legacy when I had the exact same question, Subaru's recommendation was to replace all four if the tire circumference is more than 1/4" of an inch larger compared to the other tires.

Personally, I'll likely replace all four if one of them becomes damaged beyond repair. I think the additional cost of buying four is worth the peace of mind that comes along with it.
 

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I'm just wondering if this still holds true for the RS's "unique-ish" AWD system.

From TireRack:


But we don't have a diff or viscous coupling. If the rear wheels are already over-driven via gearing I would think that tire tread level would be a minuscule factor compared to the gearing.

Continuing this train of thought, would it even matter if there was a mismatch side to side? We have an open diff up front (which wouldn't care?) and we have the clutch packs in back. The clutch packs by design are slipping all the time. It would seem to me that it wouldn't matter on the RS.
The RDU clutch packs are so sensitive to the RDU ECU that GKN can't even use "off the line" packs. Each set needs to be individually matched (left and right) to ensure that system performs correctly. As others have said, at the very least I expect you'd want to ensure that the rear tires are matched.
 

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Discussion Starter #8
The RDU clutch packs are so sensitive to the RDU ECU that GKN can't even use "off the line" packs. Each set needs to be individually matched (left and right) to ensure that system performs correctly. As others have said, at the very least I expect you'd want to ensure that the rear tires are matched.
What does sensitive mean in this context?
 

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What does sensitive mean in this context?
It means that the right and left clutch pack tolerances need to be very close. One of the GKN ECU calibrators said that clutch packs that were not precisely matched actually made the the car "crab" (pull to one side). That appears to me to mean that "sensitive" in this case means that small mechanical differences in the clutch packs are essentially magnified by the actions of the electronics. This also indicates to me that the RDU ECU is not only "telling" the clutch packs what to do, but that there are feedback measurements (from the clutch packs) that are fed into the RDU ECU algo's.
 

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I think the RS will be less sensitive than a Subaru due to the nature of Subarus center differential.

However I will still replace all four if needed simply out of principal.

Sent from my SM-N920V using Tapatalk
 

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Knock on wood I don't get any unrepairable road hazard damage, but if I do, I'll replace all 4 or see if the new tire can be shaved. Better safe than sorry!
 

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If it happens to the stocks I'll just replace all of them. But anything after that it will prolly be at least 2 at a time
 

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The RS will use wheel speed sensors to help determine torque splits/tire. The worn tires will spin faster and the ecu may interpret that as wheel spin, moving torque to the larger tires. I will be replacing my tires 4 at a time.
 

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The RS will use wheel speed sensors to help determine torque splits/tire. The worn tires will spin faster and the ecu may interpret that as wheel spin, moving torque to the larger tires. I will be replacing my tires 4 at a time.
There is also a certain percentage of error figured into the equation. Find out that percentage and you will know how much difference in tire height will be within acceptable range that the computer will not be able to tell.
 

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While I agree with you, the common advice given is that if you have to replace 1 tire on an AWD vehicle you have to replace all of them.
It will make the diffs in a normal AWD car work a lot. I always replace the axle set if I have to replace one (well not if the other tire is 90% new or newer). I wouldn't replace all four for one having an issue on this car.
 

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I'm not saying you're wrong. I'm just saying that I know Discount Tire will tell you to replace all 4 as well as TireRack. The majority of people advise to replace all 4 (or shave a new tire down to the old tires level).
It's certainly the safest practice.. and for liability reasons every tire reseller/shop will tell you to replace all 4 in an AWD/4WD vehicle.
 

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It will make the diffs in a normal AWD car work a lot. I always replace the axle set if I have to replace one (well not if the other tire is 90% new or newer). I wouldn't replace all four for one having an issue on this car.
I only replaced two rear tires when I had a 95 eagle talon awd (eclipse gsx). It was recommended to replace all four too. I was in college and didn't have the money to replace the front tires which were good. Ran the car like that for a couple years. Eventually I did replace all 4 with a set of summer and a set of winter. Maybe I was lucky but it didn't affect the car while in my possession from 60k-140k.
 

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My brother has a 04 WRX and he always replaces 2 and rotates the tires. He has 120k+ miles in the car, drives it like a maniac and he has never had one issue.

So unless Ford says to do otherwise, this it what I will be doing.
 

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Tread depth somewhat determines replacement criteria

Depends on the difference in tread depth between old and new. You typically have 10/32" to 12/32" tread depth on new tires and tires are generally considered worn when tread depth is less than 3/32". If the tread depth of your three good tires is 10/32" or better, it obvious to just buy a new fourth tire. The question becomes more complicated with less tread depth on the three good tires. By the same token if you are in the range 6/32" or less you would probably replace all four. The real question is when you tread depth of the 3 good tires is between 6/32" and 10/32" ?


YMMV,

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