Tread depth truth?

Rizzo03

Rizzo03

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Location
Romulus, MI, USA
With the Legacy 2003 platform I have and a manual transmission, What is the truth about how closely all four tires must match ?

How many 32nds difference is acceptable?


Thanks guys.
 
I'm not sure what the actual measurement would be but if the front diff & the rear diff are constantly spinning at different speeds the centre diff will wear out. how much this difference needs to be for that to happen I dont know
 
The interesting thing about the center diff is that it is always working and even more on twisty roads, that means it should break sooner in Switzerland than Australia but I don't think that this is the case.

However, I do have 4 same tires and swap them every 5000 km (front to rear, not left to right) and haven't had any issue with the centre diff (silicone one, not the DCCD !)
 
Rizzo03 said:
With the Legacy 2003 platform I have and a manual transmission, What is the truth about how closely all four tires must match ?

How many 32nds difference is acceptable?


Thanks guys.
Click to expand...


The official word is 4% of rolling circumference.
 
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subyroo said:
The official word is 4% of rolling circumference.
Click to expand...

That sounds familiar.

So if you have 215/65/16 tyres, circumference ~2154mm, diameter ~686mm (https://www.rimsntires.com/specs.jsp)

4% smaller is 2154-86=2068mm, diameter 658mm (using https://www.onlineconversion.com/circlesolve.htm cos I'm lazy lol :raspberry:)

Difference is 28mm diameter.

Or if you have larger tyres on one axle...

4% larger 2154+86=2240mm, diameter 713mm, 27mm larger. 13.5mm deeper tread, found on muddies for example, would do this.

Personally, on an AWD I would always have the same type & age tyres on all corners :iconwink:
 
NachaLuva said:
I'm not sure what the actual measurement would be but if the front diff & the rear diff are constantly spinning at different speeds the centre diff will wear out. how much this difference needs to be for that to happen I dont know
Click to expand...

6% according to Nory at alldrivesubaru.
 
NachaLuva said:
That sounds familiar.

So if you have 215/65/16 tyres, circumference ~2154mm, diameter ~686mm (https://www.rimsntires.com/specs.jsp)

4% smaller is 2154-86=2068mm, diameter 658mm (using https://www.onlineconversion.com/circlesolve.htm cos I'm lazy lol :raspberry:)

Difference is 28mm diameter.

Or if you have larger tyres on one axle...

4% larger 2154+86=2240mm, diameter 713mm, 27mm larger. 13.5mm deeper tread, found on muddies for example, would do this.

Personally, on an AWD I would always have the same type & age tyres on all corners :iconwink:
Click to expand...

28 mm diameter difference is quite a lot of difference.

Now, the 225-60-R17s have 86.8" circumference. That is 83.4 at 96%. Diameter is respectively 27.63 and 26.55 (using the same https://www.onlineconversion.com/circlesolve.htm). That is over 0.5" of a difference where one measures the tread. That is a lot of tread.

By the way, I read carefully the exceedingly boring Subaru manual, which is clearly written more as a precaution against legal action rather than as something meant to be user-friendly. It says nothing at all about tread depth. All it tells us is to use the same tire type. The goal of the whole section is to make sure people, and given the way everything is repeated over and over, apparently inanimate objects too, understand that mixing random brands and tread designs is a bad idea. That is all, there is no concern with equal tread depth. Obviously, tire shops are quick to say, AWD--change all! But, aren't they a bit too interested in doing that?

I am not arguing here that we should start driving tires of mismatched tread depth but apparently small differences in that regard should not impact the car, which means that having a non-rotated fifth tire is not that big of a problem--say I can buy it shaved to 8/32 which should cover me in all cases (new the GeoL ATs come at 12/32 and I would replace them at 4/32). Just thinking aloud here, thoughts?

OK, THE MANUAL CLEARLY ASKS FOR THE SAME CIRCUMFERENCE AND WEAR. While I bet this is a precaution, Subaru has definitely covered its back here.
 
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Rereading this, 4% or ~28mm is a lot!. I wouldnt be happy driving around with tyres 1" bigger on one axle than the other. There's way too many people who have had to replace their centre diff which aint cheap!

I also am very careful with matching air pressures as this changes the diameter as well. Important on the front for braking/steering, important on the back for the LSD
 
Well, I took a better look. Subaru does indeed speak of four tires of equal circumference and even wear so they do indeed tell us to change all four.
 
Rizzo03 said:
Who's this Nory guy ? Hey if you guys trust him then WT right.
Click to expand...

Yes, Nory does know his stuff alright. Nory was a senior tech at Subaru before joining alldrive; he used to go to Subaru Japan to train others!
 
I was forced to use my original spare at Birdsville when the other original tyres were around 50,000km and travelled more than 3,000km before I junked the lot and bought a new set (and definitely not Geolandars lol) At 250,000km the centre diff seems unharmed from the experience. So there must be some tolerance.
 
Just wondering about tyre sizes for my spare. I'm running 215/65r16 on all 4 driving wheels with a half-worn 215/60r16 spare.

Specification Sidewall Radius Diameter Circumference Difference
215/65-16 5.5in 13.5in 27.0in 84.8in 0.0%
215/60-16 5.1in 13.1in 26.2in 82.2in -3.1%

So, accounting for my half-worn 215/60r16 spare, I'm still within 4% difference in circumference between the tyres. Would this difference harm the front/rear differentials? Distances between tyre shops aren't high in NZ, so I'd probably be doing up to 80kms tops on the undersize spare.

Also, I would need two undersize tyres on the same axle to harm the centre diff, correct?
 
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Another way to do it is to over-inflate the smaller tire or to under-inflate the bigger tires. If the distances from ground to center of the wheels are the same, you're ok.
 
I plan on soon having a set of 225 65 17 GeoL AT s, and a 225 60 17 GeolL AT S spare from my old set. In the US, Subies come with a pseudo spare that is one and a half inch less in diameter than the tires, with which it shares nothing else either, be that width, tread, or even brand. Thus I figure that I will have a better spare even from a center diff point of view. Of, course it will only be a temporary spare, just as the factory donut. But it will be a proper AT tire.
 
Yeah, my spare is a brand new Geolander G900. It should be fine then.
 
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