2023 7th Gen Subaru Legacy Limited tire depth tread question...

[Wolfy4Me]

(225/50r/18 tires, 20,00 miles) I'm new to Subaru and this forum and have been reading (ad nauseam) numorous threads on how different tread depths on each tire can negatively effect our 4 wheel drive system. I am aware that Subaru states the variant should not be more than 2/32" of each tire. I've read way too many opinions and reason's to change all four tires when necessary and agree with most of them. My tires currently have 3 tires at 8/32", with one having 9/32". I'm not contemplating buying new tires, but am just curious what other's on this forum have experienced with matching their tires.
(As a side note: I took my car to our local Subaru Dealer to have the brake fluid changed and system flushed with an overall inspection. The inspection, by a "Master Mechanic", included a tire tread depth check. 10/32's on all 4 tires was documented by him. When I got home and thoroughly read my inspection print out, I decided to recheck each tire for tread depth, being they were originally rated at 9/32's when brand new. I'm definately not impressed with this dealership here in SW Missouri after my first visit with them, so I'm currently looking for a local shop that I can build some trust with).
I'm new to Subaru and have always followed the manufacturer's maintenance schedule to the letter on my previous cars, so any real world advice would be appreciated. Thank you.

 

[bmart]

Subarus use a coupling between front and back drives (truly left to right also). If the tires aren't "close enough" in diameter (also affected by a big delta in PSI), it causes the coupling (think of it like a clutch) to slip. If it slips enough, it overheats and grenades itself. Pics of mine attached after the tire place caused my grenading.

It isn't the tread depth that causes it, but varying diameters. For instance, if your tires are 1/2 worn and you need one new one...you're buying four new ones, or one used one from eBay with similar tread depth on the identical tire model.

Tread depth is a funny thing as it isn't equal across the tread. The edges look more worn far sooner than the centers as the edge tread depth is much shallower to start with.

Buy a gauge and stay on it yourself (depth and cold PSI).

Super cheap: https://www.amazon.com/Deapth-Meassurement-Thread-Trucks-Double/dp/B0C27PRJP1 or https://www.amazon.com/Tire-Tread-Depth-Gauge-Motorcycles/dp/B0DL87TQZD?th=1

Slightly less cheap: https://www.amazon.com/Milton-Industries-EXELAIR-EX448DIG-Digital/dp/B01GQ7KEG0 or https://www.amazon.com/SAFELIFE-Tread-Depth-Professional-Motorcycle/dp/B0CZCYV5QQ

 

[Wolfy4Me]

Thank you for taking the time to respond to my post. Your reply was excellent and very informative. Great pics too!. Cheers!.

 

[manicmechanic007]

(225/50r/18 tires, 20,00 miles) I'm new to Subaru and this forum and have been reading (ad nauseam) numorous threads on how different tread depths on each tire can negatively effect our 4 wheel drive system. I am aware that Subaru states the variant should not be more than 2/32" of each tire. I've read way too many opinions and reason's to change all four tires when necessary and agree with most of them. My tires currently have 3 tires at 8/32", with one having 9/32". I'm not contemplating buying new tires, but am just curious what other's on this forum have experienced with matching their tires.
(As a side note: I took my car to our local Subaru Dealer to have the brake fluid changed and system flushed with an overall inspection. The inspection, by a "Master Mechanic", included a tire tread depth check. 10/32's on all 4 tires was documented by him. When I got home and thoroughly read my inspection print out, I decided to recheck each tire for tread depth, being they were originally rated at 9/32's when brand new. I'm definately not impressed with this dealership here in SW Missouri after my first visit with them, so I'm currently looking for a local shop that I can build some trust with).
I'm new to Subaru and have always followed the manufacturer's maintenance schedule to the letter on my previous cars, so any real world advice would be appreciated. Thank you.

Much Ado about nothing!
Anytime you are on a curve all 3 differentials work or have bias
They sell more tires that way
Unless you are running 16 inch on the front and 17 on the rear you are okay (for thousands of miles)
Subtle differences like tires worn out and needing one tire is no real problem at all

 

[bmart]

Ahhh, no. Occasional changes while you're driving build up some heat (like slipping a clutch), but the system heats and cools while being used as designed. Engineers are involved for a reason. Tires that are close are fine as PSI also changes a bit. Putting one new tire on with three worn ones of significantly different size gets you a guaranteed few thousand dollar repair bill. This is well known and documented on AWD vehicles. Russian roulette is (still) not a smart game to play, like motorcycling without a helmet because...hardly anyone actually uses it.

 

[manicmechanic007]

As told by the tire boys and the chicken little trans boys
The bias is adjustable on some of the hotrod Subaru models and canyon driving does the same exact thing to the differentials
I drive 2 Mercedes 4Matics and the tire boys say the same thing
They are being told to say that by the bean counters
Do the math
The front tires take a bigger arc than the rear tires in a turn (as well as the outside front tire takes a bigger arc)
Ackerman steering angles, look it up
You are saying to go straight for the cars entire life

 

[bmart]

Thanks. The Subaru engineers will be grateful... Unusual to have two of you guys on a forum.

 

[keith3267]

Not sure how you are measuring your tread depth but the tread depth in the same groove can vary around the circumference of the tire, especially if the tire has been out of balance for a while.

My preference for a tread depth gauge is the super cheap one in bmart's post. Work fine, last long time.

Your owners manual gives you the limits, mine says 1/4" circumference, it does not say tread depth. But 1/4" circumference translates to just over 1/32" in radius (tread depth).

Not all AWDs are sensitive to differences in tire circumference, especially those that have a separate center differential as opposed to one built into the transmission. Subaru's ARE sensitive to differences in tire circumference.

It is true that in a corner, your tires spin at different speeds, but what percentage of your driving is spent in corners? In a Subaru, the center differential is different than the drive differentials and is subject to overheating if it is constantly getting two different speed inputs. It will heat up a little on a corner but cool back down in the straightways.