Hi Scotty,
I have a 2019 Subaru Legacy 3.6r, I recently had a brake fluid change at the dealer and they said they noticed 2 of the tires weren’t matching on tread depth and will cause diff damage and can wreck the trans. How true is this? Will 1mm of difference cause this.
The spec I've read for Subarus is that the tires must be within 2/32" tread depth of each other. (That's about 1.6mm.) If you need to replace a single tire there are shops that can shave a new one to match what's on the car.
This is one of the disadvantages of vehicles with full-time all-wheel-drive that people rarely think about.
true, but it takes more than 1mm
Look, you buy two identical tires with dates stamp on the sidewall indicating same production date - and presumably same production run, … and you most likely will have diameter of the two tires differ by far more than 1 millimetre. So, I think this thing given to you makes not much sense.
As to a tread depth, I’m sure if you take a new tire and measure the depth, … say, in six random places around a circumference,… you will find that depth will vary by probably more than 1 millimetre.
Further, if you take identical tires and inflate one to, … say, 30 psi and the second one to, … say, 33 psi, you will find that diameter of each tire will be different by more than 1 millimetre.
And so, what I’m saying is that if 1 millimetre variance in depth of tread would destroy a differential in an average 4WD stupid car, we would have massive failures all around, … all the time! Well, we don’t. Subaru does not, …
Now, … I’m not saying that different tire diameters are not detrimental to the life expectancy of differentials, but surely, a much greater tolerances than 1 millimetre in tread depth are more than acceptable.
Anyway, … this whole thing brings forward an issue of tire rotation. How often are you rotating your tires? The fact is that cars are not exactly symmetrical, … or equally loaded, … or equally driven into corners left and right, … or equally subjected to even braking forces front to back. This means that tire wear unevenly at each corner, … and that’s why we rotate the tires, … and that’s why we rotate 4WD equipped tires more often, … to distribute wear evenly among the four tires. FWIW.
F.S.
I just checked with Subaru, here what they said:
Measure circumference of a properly inflated tire - door jamb pressure. The four tires should have a circumference variance of no more than 1/4-inch of tire circumference. All four tires should be of the same manufacturer, same size and same model and be of a same age.
This means that if your tires are identical, average taken tread depth should calculate to something not much more than 2/32 of variance. They suggested that one should rotate Subaru tires regularly and monitor different corner wear. If you drive aggressively, half the distance travelled between recommended rotation.
F.S.
I've heard gear shops say 1/4" too