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Old 05-07-2014, 09:30 AM   #1 (permalink)
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mis matched tires on an awd

i went camping with a friend this weekend and while driving up a access road he had a sapling stump go through the side wall on his forester,

he was complaining about having to buy four new tires,

his tires look like they have 80 % tread left.

i know in theory on an awd the tires should be replaced together, but in this case it just seems overly cautious.

since most of my cars cost less than a new set of tires i know what i would do but it is not my car.

would love your thoughts, what if there was only 20% tread left? does it really matter?

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Old 05-07-2014, 09:38 AM   #2 (permalink)
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He only has to get one tire. The thing is equipped with differentials after all.
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Old 05-07-2014, 09:53 AM   #3 (permalink)
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No if he replaces one tire he'll get torque bind also you'll want to make sure that you get the same tread pattern if he replaces only one and you'll have to have it shaved to match the other tires. The problem is that with an awd system is that it's looking for wheels slip so if one tire is bigger even slightly it has a different rotation speed that the awd system is constantly trying to compensate for and will toast the trans and front diff. The price of a trans and diff replacement vs 4 new tires it's a pretty simple decision.
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Old 05-07-2014, 10:03 AM   #4 (permalink)
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what if he just buys the 1 new tire and runs it at 30 psi instead of 40?
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Old 05-07-2014, 10:35 AM   #5 (permalink)
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http://www.justanswer.com/subaru/36w...ing-tires.html

But if it's going to keep him up all night, he should just do what he thinks he has to do.
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Old 05-07-2014, 10:40 AM   #6 (permalink)
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Quote:
Originally Posted by backpacker3 View Post
No if he replaces one tire he'll get torque bind also you'll want to make sure that you get the same tread pattern if he replaces only one and you'll have to have it shaved to match the other tires. The problem is that with an awd system is that it's looking for wheels slip so if one tire is bigger even slightly it has a different rotation speed that the awd system is constantly trying to compensate for and will toast the trans and front diff. The price of a trans and diff replacement vs 4 new tires it's a pretty simple decision.
Are you saying Subies AWD is just like driving around in a 4WD with the transfer case and differential(s) locked up???

The front wheels travel farther than the rears; even if all four tires were perfectly 100% diameter matched, this would be true and the front/rear power split needs to accommodate it.
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Old 05-07-2014, 12:23 PM   #7 (permalink)
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so long as the tire is the same type and has the same tread problem its not going to be a problem. Yes the differential will spin a little. Yes he will notice a very slight drop in mpg. But if the other tires are not very worn the and the difference in circumference is not very great he will not burn out his diffs.
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Old 05-07-2014, 01:32 PM   #8 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Are you saying Subies AWD is just like driving around in a 4WD with the transfer case and differential(s) locked up???

The front wheels travel farther than the rears; even if all four tires were perfectly 100% diameter matched, this would be true and the front/rear power split needs to accommodate it.
I reread the op and it's 80% tread not 20% in that case it should be ok but subaru says that there should be no more than a 1/4 inch difference in circumference and ideally less than that.

It's not like locked diffs the problems is that the awd system keeps track of how the wheels turn and when one is larger or smaller it seems like that wheels is slipping to the awd system and it's constantly trying to correct it but it eventually will either burn out solenoids or cause mechanical failure. Google torque bind in subarus for the more technical explanation of it though
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Old 05-07-2014, 02:01 PM   #9 (permalink)
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I did it without a problem.

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