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Discussion Starter #1
I'm new to both Subaru (2018 Outback) and AWD in general. Trying to get a little more educated.

I've read several threads about the need for all four tires to be within a small margin of the same circumference, and that damage to one tire may require replacing all four.

Do all AWD vehicles have this requirement? If not, what is it about the Subaru system that requires it and what advantages does that offer?

Any info or pointers to info would be greatly appreciated.

Thanks!

Tom
 

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I simply cannot abide useless people.
2006 2.5i and 2002 3.0 wagons.
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This is common for almost every AWD system.
 

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03 H6 OBW & 06 WRX Sportwagon
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all GOOD AWD systems probably have similar requirements. A buddy here at work has an old-ish Audi - same deal.

the system needs to be sensitive to wheel slip. So, if a wheel is turning faster than the rest , FOR ANY REASON, the AWD system tries to redirect power. On actual slippery surfaces, there is no problem. The systems does not 'know' you have lost traction, only if a wheel is turning faster than the others.

BUT, if a wheel is smaller than the rest, or the rest are 'detected' as small due to a larger wheel in the mix, and you are on dry pavement, the re-directed power can build-up as stress in the drivetrain.
 

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pretty typical of any AWD system,

if you want to run mismatched tires, drive a old RWD SUV or truck that you turn the fronts on occasionally for 4WD,
like when it can't go no more (such as when the ass end is sinking into a muddy lawn).
 

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2005 Outback 3.0R VDC/VTD/LSD 5eat , 2.8'' lift
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and tires are not only problem there. everything that AWD system uses to keep your car safe must be in perfect order, brakes, steering wheel and all components that goes into that. even faulty knock sensor was failing on my VDC system. but thats good thing . with that you know that you drive safe car .
this could make all more clear
 

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I've read several threads about the need for all four tires to be within a small margin of the same circumference, and that damage to one tire may require replacing all four.

Do all AWD vehicles have this requirement?
The one-size-fits-all public consumption answer is "yes".

The actual answer is "no".
 

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Discussion Starter #7
Thank you all for these responses. I "get" it now. I had been wondering why the computer control system could not be reset when you change tires and set up for the actual circumference of the individual tires. However, is that not possible because the differentials on each axle are mechanical?
 

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yeah, there is a 'final drive ratio' that is mechanically the same at each end of the car, and at each wheel.

maybe someday in the future there could be an acoustic, tactile, visual, or other....? method for a system to actually detect slippage at the rubber:road surface interface - but at present, rotation is the indicator.
 

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On AWD systems with a 'tight' mechanical coupling, like Subaru and Audi, the answer is that 1/4" circumference difference max (approx 2/32" of an inch tread depth +/-) is a good rule of thumb. These systems are pretty much full time AWD, although they may vary the % torque split front to rear as you drive.

"On Demand" systems such as used on the Sienna minivan, Honda CRV, Nissan Rogue are FWD much of the time, and only invoke the rear axle a percentage of total driving time. As the front differential is open, and the rear is not powered all that much, you can get away with a lot more 'abuse' before you damage something. Still, there are limits to what you can do if you don't want to be replacing internal clutches.
 
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Our dealer can "shave" a new tire if just one needs to be replaced. This is an option instead of replacing all four tires due to one being blown out or some other type of tire failure.

I would opt for this dealer service if all four tires still had a lot of life left in them over buying 4 new tires because one needed to be replaced.
 

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Discussion Starter #11
I wasn’t thinking beyond the speed sensors (Re the mention of acoustics etc. ) it just seems that the computer is comparing the readings from the wheel speed sensors and assuming the wheeel are all identical, ie, the weighting factor is a default 1 for each. I was thinking that if one tire is smaller, the rotation factor could like 1.02. A smart system would thus be able to factor different size tires when making the rotation comparisons. That’s why I asked about the mechanical component. That would be fixed.
 
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