Need to replace all four tires on AWD?

[SkipII]

I understand the impact of differing circumferences on how AWD works but I am trying to get a better idea of how sensitive it is.

 

My wife hit some debris on the road this weekend and blew out two tires on her Legacy GT. The car has Bridgestone Potenzas with about 10K on them.

 

The dealer says all four tires need to be replaced. I say move two of the tires in a pair to either the front of the back (whichever normally creates greater wear) and just be cool for a few months until they even out.

 

I have a hard time believing that 2/32 or so of tread wear after 10K would make that much difference to AWD.

 

Any opinions?

[Manarius]

I'd replace all 4. I wouldn't want to take any chances with the AWD system. $400 for new tires or $1500+ for a new tranny if you screw it up.

[tcspeer]

Take this how you want, but I put 310,000 miles on my Legacy postal Subaru before torque bind finnaly set in on my car. I did not replace my tires in sets I went through way to many tires for that. However I bought a 98 Legacy wagon from another carrier for 100.00 dollars because the transmisson was shot. After taking the transmission out I noticed it had a remanufacted tag on it, so this was his second transmission after replacing it with one from wrecking yard I noticed he was running different size tires on the car. So I put a new matching set on it and have not had any problems with it. When it needs a tire I will buy one tire not a set. If this car wont hold up under these conditions then I will sell it and get one I can use. I am not telling you this is the correct way that you must decide, but I saved enough money on tires the last ten years to buy another transmission from the wrecking yard.

I understand the impact of differing circumferences on how AWD works but I am trying to get a better idea of how sensitive it is.

 

My wife hit some debris on the road this weekend and blew out two tires on her Legacy GT. The car has Bridgestone Potenzas with about 10K on them.

 

The dealer says all four tires need to be replaced. I say move two of the tires in a pair to either the front of the back (whichever normally creates greater wear) and just be cool for a few months until they even out.

 

I have a hard time believing that 2/32 or so of tread wear after 10K would make that much difference to AWD.

 

Any opinions?

[otis]

Any opinions?

Well, since you only asked for opinions, here's mine...

I don't think it would make a difference. if it were my car, I'd put the 2 new tires in front and be done with it. my wife had a 96 imprezza (5mt) where in the very first year, she got a blow out. every year or so, she'd replace another tire. this went on for 80k miles with no apparent problems. she never replaced any tires in pairs, let alone sets of 4. our current 00 forester (auto) got a flat in one tire and we replaced it 2 years (25k miles) ago with a used tire. turns out the used tire (different tire brand) had a much bigger circumference (I think it was > 0.5"). we finally replaced all 4 tires just last week. add to that the post above and you get 3 seprate incidences on the internet, so you HAVE to debunk this myth.

 

actually from a statistical standpoint, this anectodal evidence means nothing. but at the same time, I do recall posts about folks who alway rotate their tires and get torque bind as well as people who claim their transmission problems stem from mismatched tires. I have yet to see a post that says they see a real corrolation between tires and transmissions.

 

Nobody knows the answer. everyone is going by the spec in the manual. there are those who believe that the failure is real but the 0.25" tolerance is very strict. then again the manual also says get an oil change every 7500 miles, and everyone here changes it at their own interval. my bmw manual says my transmission fluid is lifetime and I never need to change it.

 

SO change only 2 tires at your own risk. my opinion is in the minority here. but then again, I did (on 2 separate cars), and we're ok.

[SkipII]

 

Nobody knows the answer. everyone is going by the spec in the manual. there are those who believe that the failure is real but the 0.25" tolerance is very strict. then again the manual also says get an oil change every 7500 miles, and everyone here changes it at their own interval. my bmw manual says my transmission fluid is lifetime and I never need to change it.

 

SO change only 2 tires at your own risk. my opinion is in the minority here. but then again, I did (on 2 separate cars), and we're ok.

 

Thanks. Bet out -- if the tranny binds, and they see I have different treadwear, I'm sure I'll be in a battle.

 

By the way, I own a 330Ci and had my auto tranny fluid changed at 45K (the delaeer claimed they had to crack open the tranny casing to do it since there was no way!). Easy to do -- cut the tube from the tranny cooling system and installed a fitting. "Lifetime" fluid simply means some statisician and run the numbers to project how many fluid-related failures there would be within warranty and then within typical CPO periods. It has NOTHING to do with the fluid being lifetime. it is merely synthetic, which has longer change intervals.

 

Thanks for your help. I'll cough up for a pair of Potrenzas and tell my wife to stay away from construction zones.

[OB99W]

I understand the impact of differing circumferences on how AWD works but I am trying to get a better idea of how sensitive it is.

 

My wife hit some debris on the road this weekend and blew out two tires on her Legacy GT. The car has Bridgestone Potenzas with about 10K on them.

 

The dealer says all four tires need to be replaced. I say move two of the tires in a pair to either the front of the back (whichever normally creates greater wear) and just be cool for a few months until they even out.

 

I have a hard time believing that 2/32 or so of tread wear after 10K would make that much difference to AWD.

 

Any opinions?

You know what they say about opinions, so I'll try to provide as many "facts" as I can, some opinions, and let you decide for yourself.

 

1) Subaru says tires need to be matched within 1/4 inch of circumference. Based on what you said (2/32" of tread wear, which is the radius change), your tires would mismatch new ones by more than 12/32" (3/8") in circumference, which is 50% over the max. Info relating to possible consequence: http://www.endwrench.com/pdf/drivetrain/RearAxleBindingInfoF99.pdf

 

2) Unless you manage to get exactly the same replacement tires as the ones now on the car, they may not be identical in circumference to the used ones (as when new). That could be good, if they're somewhat smaller than the originals were, or worse, if they're bigger. OPINION: Tire dealers used to "shave" tires in order to true/match them. You lost a little rubber, but gained better rolling. More recently, this service seems to be less available at tire shops, but race shops probably still do it. Having 1/32" or so of tread depth removed from the new tires to more-closely match sizes might be worth considering. (Assumes you determine the actual difference; inflate new and old tires to the same pressure and measure accurately before considering this.)

 

3) The AWD system requires a match in front-to-rear size. In actuality, a left/right mismatch, if nearly identical at both front and rear axles, doesn't affect the AWD. The size difference is "absorbed" by the front and rear differentials. That would mean placing the used tires on one side of the car (rather than front or rear) would be better for the AWD. OPINION: However, even if the new tires have identical tread pattern to the used ones, from a handling/safety standpoint I'd prefer to have left/right matched, with the better (newer) tires on the front.

 

4) OPINION: A few pounds more inflation pressure in the older tires than the new ones can help even out the effective radius, and should minimize the possible problem of size difference.

 

5) OPINION?: Front tires usually wear faster than rears, due to various factors including cornering and braking forces. However, they don't typically wear as evenly as the rears.

 

6) Previous discussion (one of many; includes some facts, lots of opinions):

http://www.ultimatesubaru.org/forum/showthread.php?t=58302

[elmwood22]

Look at it this way. You still have three good tires, correct? Sell the three at a slight discount, being that they have 10k on them you should get some good money for them. Also, what type of mileage tire are they, this would aid in getting more money for them? Then go and purchase 4 new tires for your suby. Problem solved. You figure a decent set of tires will run anywhere from $350-550. At least by selling the remaining three you could help your cause.

[jon38iowa]

I'd replace all 4. I wouldn't want to take any chances with the AWD system. $400 for new tires or $1500+ for a new tranny if you screw it up.

My two cents: I agree with the above, moreover I find getting some parts for Subaru a bit a challenge and rather expensive. Four new tires appear to be cheap insurance to me.

John:brow:

[nipper]

2 new tires go on one side of the car, two older hte other. SUbaru spec is 1/4 inche cricumfarance between tires. This way you are splitting the difference.

 

 

nipper

[nkx]

you could always have the new tires shaved to the tread depth of the old tires.

[jct61765]

 

1) Subaru says tires need to be matched within 1/4 inch of circumference. Based on what you said (2/32" of tread wear, which is the radius change), your tires would mismatch new ones by more than 12/32" (3/8") in circumference, which is 50% over the max.

 

2/32 or 1/16 of an inch is alot less than 1/4. Its still 3/16 under the 1/4 subaru calls for mismatch. He should be fine.

[OB99W]

2/32 or 1/16 of an inch is alot less than 1/4.

True, but not relevant.

 

 

Its still 3/16 under the 1/4 subaru calls for mismatch.

Relevant, but untrue, according to Subaru. The 1/4" maximum mismatch allowed is in circumference; I previously gave this link, but here it is once more: http://www.endwrench.com/pdf/drivetrain/RearAxleBindingInfoF99.pdf

 

As to the math; the 2/32" tread depth wear is a loss in radius. Circumference, however, is calculated by multiplying radius by "2 pi".

Pi is a bit over 3 (3.14159...), so 2 pi is somewhat over 6. That means 2/32" in radius change is a circumferential change of over 12/32" (which reduces to 3/8"); that's 50% over the 1/4" (which is 2/8") allowed. See: http://mathforum.org/dr.math/faq/formulas/faq.circle.html

 

 

He should be fine.

He may or may not be, which is why I tried to provide enough info so that an informed decision could be made.

[SkipII]

Thanks everyone, and particularly OB99W for offering and separateing fact from opinion.

 

It sounds like I could have left two tires on one side and bought two new ones for the other side, but we are already into getting this done at the dealer. I will keep the two "good" tires and use them someday if we have another situation (ideally in 10K miles!).

[OB99W]

Thanks everyone, and particularly OB99W for offering and separateing fact from opinion.

Glad to provide what I could.

 

 

It sounds like I could have left two tires on one side and bought two new ones for the other side, but we are already into getting this done at the dealer. I will keep the two "good" tires and use them someday if we have another situation (ideally in 10K miles!).

It probably worked out for the best, at least in terms of possible imact on the AWD (yeah, monetarily it's another thing ). Let's hope you don't need to replace another tire or two in 10k miles, because the reason for that usually isn't anything good.

[johnceggleston]

). Let's hope you don't need to replace another tire or two in 10k miles, because the reason for that usually isn't anything good.

 

now go buy 2 used rims, and have the old tires mounted. then, when you've run the new ones 10k miles , rotate in the 2 spare wheels with tires. now 6 tires are working.

 

if you buy 4 rims, you'll have a snow tire set after you wear out these 6 tires. my wife used to detsroy 1 tire a year on the mini van. good thing it wasn't awd.