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Discussion Starter #1
I'm looking at the possibility of buying a used set of Michelin PS A/S tires with about 10k of wear on them. According to the seller, the pair that came off the front has 5/32 of tread left and the rears have 8/32 left. Anyone know if this will be within tolerance for the AWD system, or will it cause problems? :think:
 

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Can't answer until you tell us the tire size.....
 

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They would be way out of tolerance. The spec of 1/4" difference in circumference equals just over 1/32" in tread depth, independent of tire size.

I've cheated by a bit in the past with no adverse effect, but never with this much of a difference.
 

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Depending on size, they may just be in or out of spec.
 

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Just for example New 225x55x16 with tread depth of 10/32, Circumference = 80.771

Worn 225x55x16 with tread depth of 8/32, Circumference = 80.3785

So this would NOT be within the .25" circumference rule.

Still need to know the size to determine if it's within spec.
 
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I wouldn't buy the tires. The ones with 5/32 of tread left are almost worn out. Most experts recommend scrapping tires with 4/32 or less of tread left. For example, here's what the tirerack.com tech page says:

"According to most states' laws, tires are legally worn out when they have worn down to 2/32" of remaining tread depth. To help warn drivers that their tires have reached that point, tires sold in North America are required to have indicators molded into their tread design called "wear bars" which run across their tread pattern from their outside shoulder to inside shoulder. Wear bars are designed to visually connect the elements of the tire's tread pattern and warn drivers when their tires no longer meet minimum tread depth requirements.

However, as a tire wears it's important to realize that the tire's ability to perform in rain and snow will be reduced. With 2/32" of remaining tread depth, resistance to hydroplaning in the rain at highway speeds has been significantly reduced, and traction in snow has been virtually eliminated.

If rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32" of remaining tread depth. Since water can't be compressed, you need enough tread depth to allow rain to escape through the tire's grooves. If the water can't escape fast enough, your vehicle's tires will be forced to hydroplane (float) on top of the water, losing traction.

If snow covered roads are a concern, you should consider replacing your tires when they reach approximately 6/32" of remaining tread depth to maintain good mobility. You need more tread depth in snow because your tires need to compress the snow in their grooves and release it as the they roll. If there isn't sufficient tread depth, the "bites" of snow your tires can take on each revolution will be reduced to "nibbles," and your vehicle's traction and mobility will be sacrificed. Because tread depth is such an important element for snow traction, winter tires usually start with noticeably deeper tread depths than typical All-Season or summer tires. Some winter tires even have a second series of "wear bars" molded in their tread pattern indicating approximately 6/32" remaining tread depth to warn you when your tires no longer meet the desired tread depth."
 

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If the spec is always 1/4" circumference, then size doesn't matter. A quarter inch in circumference translates into just over 1/32" in radius (tread depth), regardless of size.

If however the spec varies with size, then you're right. Size matters.

Let's see ... the thing that would bother the AWD is a difference in angular rotation. A larger tire would consume less of an angle per unit circumference traveled, so it looks like size does matter.

However, if 1/4" is the circumference spec for the standard 16" tire, then the spec should increase in proportion to an increase in tire size. I wouldn't think that the percentage difference in circumference between a 16" tire and an 18" one would be enough to make too awfully much of a difference in the spec.
 

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Discussion Starter #8
Sorry, forgot to include the size... 225/50/17 :wiggle:

I guess I'm asking for trouble with this much of a difference in wear.
 

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ferret said:
Just for example New 225x55x16 with tread depth of 10/32, Circumference = 40.385714
Circumference? This implies a diameter of about 13".
 

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Yes that stands corrected..I will correct that post..I used Radius for diameter, then post Silky's answer in the next post.
 

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New Michelin P/S A/S 225/50/17 (tread Depth 10/32) has a circumference of 82.0285

Worn to 8/32 has a rolling diameter of 81.63.57

Worn to 5/32 has a rolling diameter of 80.0464

Out of the 1/4" or .25" Subaru Spec.... I would pass on these.
 
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I think your 5/32" figure should be 81.0464, but any way you look at it she would be well out of spec.

Even more telling than our attempts at math is jmacmaster's observation that a tire that's down to 5/32" isn't really a very good tire.
 

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Discussion Starter #13
Yeah, that doesn't seem like such a good idea after all... thanks for doing all the research, guys!
 
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