Not an issue in my book other than the fact your out of sync in the tire dept. Next go round with tires you may have to bite the bullet and buy 4.

 

Thanks for the input. and yes agree , when the two wear out i am 100% replacing all 4.

I should note that the tires are of different brand as well.

 

I seriously doubt that a 3/32" difference in tread is going to matter. With the expected range of normal vehicle weight loading and always some variance in tire pressure the impact of the additional tread depth is "in the noise" as far as significant effects to the transfer case, handling, etc. A full time transfer case system has to be able to allow for differential front to rear speed and someone living in the mountains would put the transfer case through a lot more compensation in normal driving than your very slight tread depth difference will create.

If you are really worried about it some tire shops are equipped to lightly shave the tread to make your new tires match your old. Otherwise if you are really trying to match things up you could inflate the older tires SLIGHTLY above recommended base pressure and achieve the same correction. But if it were mine I wouldn't bother doing either!

 

I also agree it won't matter. If it did what would happen when people with AWD and a spare tire and put it on. Virtually no chance it would match even across the axle or front to back.

Then relying on every owner to rotate their tires exactly when needed and then to buy as a complete set of four would never happen.

 

I don't know about the ATS AWD, but I have a Subaru Forester and it's very important to keep the tire circumferences within tolerances. I think there can be no more than a quarter inch variation in circumference, otherwise damage will occur. That's why they emphasize that tires be rotated religiously. Even inflation pressure to factory specs is very important from that standpoint.

 

Rocket,

If the Subaru requirement is that critical they really should include an accurate simultaneous display of the RPM of all four wheels so that the owner can set inflation pressure in order to stay within a tight spec; perhaps something like the old synchroscope display that allowed an operator to very accurately manually sync a generator to a power line before combining the two. I have a feeling they set the standard very tight in hopes that people would stay within reason for tire differences. I assume the maximum 1/4" difference requirement must be for diameter which equates to about a 1 percent tolerance for 25 inch diameter tires, if it truly is circumference then the required tolerance is around 0.3 percent which far exceeds most peoples' ability to carefully measure.

If Subaru wanted to get out of warranty service for failed AWD components I expect they could show that most customers are not within spec based upon specific corner loading, PSI, and resultant diameter and if they are in spec now then a short drive with heavy braking will provide different results since the front accounts for the lion's share of the braking and this heat will be transferred to the wheel impacting front tire psi more than the rear. And this is the perfect introduction for one of the nitrogen sales folks to appear with a solution looking for a problem

 

I'm not sure about the quarter inch quote. There has been much discussion about it on the Forester forum, but my curiosity is up so I'll check it out. I know if one tire is ruined, and it's down a certain amount in tread depth, seemingly knowledgeable people recommend replacing all four tires, or shaving the new one down. But that's posts on a forum ... not necessarily authoritative. I'll also check the owner's manual and see what it says. You're right .... the tolerances seem very tight.

 

I don't see 3/32nds being an issue, even for the Subaru. A quarter inch is 8/32nds. That would be the difference in having one set down to the wear bars or less, and one set being new.

 

Don't forget, the OP is taking 3/32 on the radius because he is only measuring one side of the tread. You are talking 8/32 (1/4") on the overall diameter. So a 1/4" on the diameter would actually only be 4/32 when measured on the tread.
Not unless the tread depth gauges compensate for the difference between diameter and radius.

 

The purpose of this bulletin is to help explain the operating characteristics of 4WD/AWD systems.

For specific operating instructions for individual transfer cases, please refer to the Owner Manual or Service Information.

DEFINITIONS
AWD vs. 4WD
The very basic difference between AWD and 4WD is the intended usage of the systems.

AWD is usually intended for on-road use in inclement weather conditions, while operating smoothly on dry pavement by allowing for a difference in speed between the front and rear axles while turning. These systems are not selectable and do not have low range gearing for the transfer case. These systems can be found in cars or trucks.

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TIRES
Tire rolling rates can be a major factor in operational characteristics. Consider the following important items:

• All tires are the same size and brand. Tires of different brands may have different circumferences (or radius) even if they are the same size.

• Tires are set to factory recommended pressures. A tire with low air pressure will roll at a different rate.

• All tires have approximately equal amounts of wear. Tires with different amounts of wear will roll at different rates.

• All tires are the same tread type. Don't mix on-off road, all-season or street tires on the same truck as they may have different circumferences and roll at different rates.

• One or more of the tires may show small, short scratches around the circumference of the tire tread. The tire "scuffing" on the road surface causes these scratches.

 

As said above, "all tires have approximately equal amounts of wear", and I am sure everyone would recommend exactly the same tires at all 4 corners, same brand, same model, same wear. When I look at the population of old Subaru Outbacks I don't see these owners having a set of relatively new tires, having to replace one and then buying a complete new set of tires. A new set of tires for some of these vehicles is around a 1/3 of the value of the vehicle so a huge investment and I believe most of these old Subaru's AWD is operating normally.

I also don't believe most people rotate their tires exactly when they are supposed to and if dealers go to rotate the tires and there is 3/32" more tread on one or two tires does he then recommend replacing all four even though all of the tires have enough tread to be safe?