I replaced one tire on my awd car

TiGeo

$50 site donor 2024
Joined
Nov 5, 2009
Messages
9,442
Location
NUNYA
So I replaced one of my Michelin Pilot Sport All Season 4s on my Sportwagen due to damage (Discount Tire certificate so it was free) before swapping out my summer tires last month. So currently my tire tread depths are:

1@10/32 (rear)
1@6/32 (rear
[email protected]/32 (front)

This is a gen 5 Haldex awd system - open diffs and a pump/clutch pack to engage/transfer torque to the rear axles at varying levels depending several variables (it's not just "when the fronts slip"..it's any time you give it gas at some level). I've of course read all of the various opinions on this but and never found any concrete guidance from VW/Audi on the max difference either front to back or on a single axle w/r to overall diameter difference that is ok.

Here's my math, I don't think 1.3% is enough to lose any sleep over...it drives normally, logging the Haldex bits is all normal as well. I think next fall when these go back on I'll probably replace 3 of them as the fronts should get down below 4 before putting summer tires back on in March and at that point they pretty well done...maybe I'll be wrong and these can get another winter out of them, I only drive about 3-4K total with these on a year and they have around 17K on them total at this point.

Screenshot 2024-12-16 140628.webp
 
Last edited:
I only found a "generic statement" as well:
1734377829616.webp


I think most AWD systems, even the notoriously picky Subaru systems, can be fine with replacing 2. This is assuming the typical scenario of two 10/32" tires on one axle and 5-6/32" on the other.

I remember working on a Subaru where the owner had not rotated the tires in a several years; the two front tires had 2/32" more wear than the rears. System had no issues.

Replacing one may be a different situation....that is unchartered territory for me.
 
Last edited:
I only found a "generic statement" as well:
View attachment 254415

I think most AWD systems, even the notoriously picky Subaru systems, can be fine with replacing 2. This is assuming the typical scenario of two 10/32" tires on one axle and 5-6/32" on the other.

I remember working on a Subaru where the owner had not rotated the tires in a several years; the both front tires had 2/32" more wear than the rears. System had no issues.

Replacing one may be a different situation....that is unchartered territory for me.
No long uncharted....I've done it and apparently....so far at least...survived! 🤣
 
Kittens will die according to the internet. (I suspect you will be fine but my OCD is talking back)
I know, I was ready for a catostrophic rear diff/awd implosion!
 
In that case, uneven power distribution should be right up your alley. Maybe the extra strain on the AWD is similar to driving constantly in a circle. Hello, NASCAR fans!
What uneven power distribution? What extra strain? How exactly are those occuring?
 
Here's a before log with 4 more evenly sized tires (still...one was replaced but these have low miles so the difference is at most 2/32" which I call "background noise/normal variance"):



Here's yesterday (withthe single larger tire that is the subject of this post):


Same stretch I always use to log with a first-fourth pull - there are a few turns here. Axle speed delta and Haldex pump activity shown. I don't see any differences...
 
Last edited:
What uneven power distribution? What extra strain? How exactly are those occuring?
Differentials do not like having to constantly compensate for differences in rotational speed. By having one wheel spin always faster or slower than the other wheels extra wear will occur in the differential. At least that's how I understand it and you may want to ask a mechanical engineer for clarification. All manufacturers of AWD vehicles give guidelines and set limits on tread wear tolerance between all tires. 5.5/32 seems too much to me. I wouldn't want that difference between left and right on a 2-WD.
 
So currently my tire tread depths are:

1@10/32 (rear)
1@6/32 (rear
[email protected]/32 (front)
...
Here's my math, I don't think 1.3% is enough to lose any sleep over
I get an even smaller difference than 1.3%
The rears are different by 4/32, doubled to consider both sides against diameter is 1/4, which is 0.98% of a 25.5 diameter.
The fronts and average rear are 4.5 and 8 over 32 respectively, difference is 0.11, doubled is 0.22, which is 0.86% of a 25.5 diameter.
Both less than 1% sounds like a nothing sandwich to me.

PS: I'll add that the Toyota GR Corolla's 4WD system is designed to constantly slip with a 0.7% difference between front & rear.
 
Differentials do not like having to constantly compensate for differences in rotational speed. By having one wheel spin always faster or slower than the other wheels extra wear will occur in the differential. At least that's how I understand it and you may want to ask a mechanical engineer for clarification. All manufacturers of AWD vehicles give guidelines and set limits on tread wear tolerance between all tires. 5.5/32 seems too much to me. I wouldn't want that difference between left and right on a 2-WD.
The difference across the rear is 10/32 to 6/32, the 5.5 is the max front to rear as the fronts are at ~4.5/32 each. I understand the concepts here and I can rationalize that the rear open diff has a rotational speed delta across it. I believe I can log the ABS wheel speed sensors on the car...I'll give that a whirl and report back.
 
Also, the ABS and traction control, and ESP may be compromised if a wheel is out of spec by more than 1.3% or whatever the common tolerance for the tire diameter is. I remember 1.3% from my dad's old Phaeton but I may be mistaken.
 
The difference across the rear is 10/32 to 6/32, the 5.5 is the max front to rear as the fronts are at ~4.5/32 each. I understand the concepts here and I can rationalize that the rear open diff has a rotational speed delta across it. I believe I can log the ABS wheel speed sensors on the car...I'll give that a whirl and report back.
Ah your 1.3% is a max corner difference - got it, that is correct.
 
The difference across the rear is 10/32 to 6/32, the 5.5 is the max front to rear as the fronts are at ~4.5/32 each. I understand the concepts here and I can rationalize that the rear open diff has a rotational speed delta across it. I believe I can log the ABS wheel speed sensors on the car...I'll give that a whirl and report back.
Side-to-side differences in tread depth worry me more compared to front-to-rear because of possible traction problems on wet roads.
 
Back
Top