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Discussion Starter · #1 ·
Well, I had a tire shred on my 2002 Subaru Forester S today as my wife was driving it. I believe it occurred because of a rim leak combined with the many potholes in our area, and then her driving it home for a few miles. The old tire is toast and not repairable, so I'm attempting to decide about getting 1 replacement or 4 replacements. Here is my reasoning:

As stated in other threads in this forum, it seems like Subaru's maximum recommendation for Tire size is either 1/4" circumference or 3/32" tread depth. I will be using tread depth since it is 9º out and I have no desire to jack up my car.

The tires on on my car currently are Michelin Primacy Mxv4 215/60R16 95V. They currently have an unknown circumference and a tread depth of 0.244" (measured to the best of my abilities with a pair of calipers in the center of a clean tire).

These same tires when brand new are shipped with a circumference of 82.18" and a tread depth of 11/32."

0.343 (11/32") Original Tread Depth - 0.244 Tread Depth remaining = 0.99 Difference in tread depth between new and old tires

3/32" MAX recommended tread depth difference converts to .094"

So, if I replace one tire on my Subaru I will exceed Subaru's max recommended difference in tread depth by 0.005." Having worked in CNC machining I fully understand the actual size of 5 thousandths of an inch and I cannot seem to justify replacing all four tires because of it. Am I being ignorant or will I really see some issues with my AWD system if the golden rule is not follow to the thousandth.

Thanks!
Jamie

The Carnage:

 

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Not sure about your math, it looks like you're stating that the old tires are 8/32 and a new one would be 11/32.....I would get the tread depth more confidently measured with a tread gage and then make the decision, likely just ordering one shaved tire from tirerack so I wouldn't fry the differentials.
 

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2014 Forester 2.5i 6 spd
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Did the 3/32 tread depth = 1/4 inch circumfrrence result from calculation? Don't recall seeing it in the manual, which is out in our parking lot. That tread depth measurement would also vary by nominal tire outside diameter. I've gotta admit I haven't checked to see the effect of the trend of larger wheel size/wider profile tires & rims on overall outside diameters.

I have to agree that 0.005 inches in circumference should be insignificant. However, should be and are can be two entirely different things. Especially since we're talking rolling circumference. I'd also endorse shaving the new tire, distasteful as that may be.

My ex-wife created a few tires that looked by that, my sympathies.
 

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Subaru backed off of the 1/4" spec a while back (coincident with introduction of the donut spare). Now there is no spec AFAIK.

My calculation says 1/4" circumference = 0.0398 tread.

Edit: ... Which equals 1.27 32nds
 

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2019 Crosstrek 2018 Forester XT
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Your math is wrong you have to multiply the 3/32 times 2 and then multiply by pi...that equates to .59" so you are way out.
 

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Discussion Starter · #6 ·
Not sure about your math, it looks like you're stating that the old tires are 8/32 and a new one would be 11/32.....I would get the tread depth more confidently measured with a tread gage and then make the decision, likely just ordering one shaved tire from tirerack so I wouldn't fry the differentials.
I totally agree with this, I'll have the guy doing my tires take a look at it tomorrow…

Did the 3/32 tread depth = 1/4 inch circumfrrence result from calculation? Don't recall seeing it in the manual, which is out in our parking lot. That tread depth measurement would also vary by nominal tire outside diameter. I've gotta admit I haven't checked to see the effect of the trend of larger wheel size/wider profile tires & rims on overall outside diameters.

I have to agree that 0.005 inches in circumference should be insignificant. However, should be and are can be two entirely different things. Especially since we're talking rolling circumference. I'd also endorse shaving the new tire, distasteful as that may be.

My ex-wife created a few tires that looked by that, my sympathies.
Sorry I didn't make the distinction clear enough, the tread depth and circumference recommendations have nothing to do with each other based on my knowledge… Thanks for the input and sympathies though, but it's not all my wife's fault, I noticed it was low again a few days ago, I should've had it checked and it may have been replaced under warranty!

Subaru backed off of the 1/4" spec a while back (coincident with introduction of the donut spare). Now there is no spec AFAIK.

I'd say that you're within experimental error. My calculation says 1/4" circumference = 0.0398 tread.
Thanks for the info, but my car is from 2002 and has a full-size spare… would that indicate the spec still applies?

Your math is wrong you have to multiply the 3/32 times 2 and then multiply by pi...that equates to .59" so you are way out.
?? again, I probably was not clear enough, my mistake. I didn't measure circumference of the tire at all, but it sounds like your trying to calculate the difference in circumference using the tread depth?
 

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Discussion Starter · #8 ·
So based on the above I might have a bigger problem than I think. I love how they say "About 2/32" as if "about" can come into play when measuring that small. I supposed I should measure tire circumference and see if I'm within 1/4" since I think that may be more accurate…

My second thought just now is that the S-model I have has a viscous LSD. Doesn't that mean that the rear tires can spin at slightly different speeds and it's not going to completely destroy everything?

Thanks,
Jamie
 

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Discussion Starter · #9 ·
Lastly, then I'll shut up. Here's what my Forester Manual says:

Wheel replacement
When replacing wheels due, for example, to damage, make sure the re- placement wheels match the specifications of the wheels that are fitted as standard equipment. Replacement wheels are available from SUBARU dealers.

WARNING
Use only those wheels that are specified for your vehicle. Wheels not meeting specifications could interfere with brake caliper op- eration and may cause the tires to rub against the wheel well housing during turns. The resulting loss of vehicle control could lead to an accident.

So if I'm acting just based on that, I am following all the rules haha. That's the only thing mentioning tire replacement of a single tire…
 

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Whether your model might or might not have had donut spare isn't the issue. It would have been hypocritical for them to have insisted on 1/4" while offering a vehicle that could clearly be in violation. So they backed off a specific number.
 

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2019 Crosstrek 2018 Forester XT
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?? again, I probably was not clear enough, my mistake. I didn't measure circumference of the tire at all, but it sounds like your trying to calculate the difference in circumference using the tread depth?
Exactly..to get 1/4" difference in circumference means .040" difference in tread depth.
But measuring the circumference (inflated) is more accurate.
 

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They say "about" 2/32" because it depends upon OD. The larger the tire diameter, the greater the difference in circumference. A tire with a 27 inch OD will create a 0.79" circumference difference with a 1/4" diameter change, a .180 change with a 0.060" (about 2/32) change. A ~2/32 tread depth difference will cause a 0.38" circumference change. Those numbers are going to get bigger as the tire OD gets bigger.
 

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I think this thread sums up how confident you can feel by running a mismatched worn tire; as such is why I said I'd just buy a shaved tire from tirerack and be done with it.

If you have a viscious LSD I'm pretty sure you'd burn the fluid by shearing it constantly with mismatch sizes.
 
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