I replaced one tire on my awd car

Emailed Borgwarner in Europe with this question.
 
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 two front tires had 2/32" more wear than the rears. System had no issues.
YMMV

Contrary to popular belief Subaru has multiple AWD systems. The most common system in Outback automatics is a 90/10 system with a viscous coupling. Those are pretty tolerant front/ rear.

Manuals have a clutch pack, The H6’s and many turbo Outbacks had the VTD drive system, which has a center planetary differential.. Would not chance it with those.
 
YMMV

Contrary to popular belief Subaru has multiple AWD systems. The most common system in Outback automatics is a 90/10 system with a viscous coupling. Those are pretty tolerant front/ rear.

Manuals have a clutch pack, The H6’s and many turbo Outbacks had the VTD drive system, which has a center planetary differential.. Would not chance it with those.
The common CVT cars all have clutch packs as a centre diff, and the common manual cars have a centre diff with viscous coupling to transmit some power to the axle with grip.
My WAG for the CVT cars, is that given subaru OK's running a very much smaller spare tire on the rear axle for a short distance, is that the subaru system is designed to ignore a rear axle overspeed situation since its impossible to happen under power given that the car doesn't have a centre diff. Maybe they have some logic built in, to also ignore the one rear tire spinning faster than the other, if the combined rear axle speed is faster than the front axle?
On our car you have to put the spare on the rear axle, so if you have a front flat you have to put a good rear up there and the spare on the back axle.

I would think for a totally gear/diff based system like on the sportwagen, as long as you don't trigger any of the difference in wheel speeds reactions, and don't put the drivetrain under heavy loads(like towing a parachute of a trailer, or a steady 120mph down the autobahn), you can get away with tires at the extremes of tread wear differences without a problem.
 
My FIL had to get a new rear diff in his '04 CRV. He was notorious for not rotating tires and was on the it's cheaper to by 2 and replace instead of 4. He wore the fronts down to about 2/32" while the rears had at least 8/32" if not more. Then he started getting a lot of odd noises and binding.

Granted totally different system with full mechanical. Front wheels spin and engage some piston/pumps in the rear. He was quite "happy" when he had to pay out a large chunk for the diff and labor AND got 4 new tires.

Pilot had a non fixable flat last year, I was fortunate I found a matching used one at the same depth on ebay. This will be the last season on them probably anyway.
 
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.

View attachment 254409
When zi worked for a VW dealership the service manager and mechanics always said no more than 2/32nds side to side. Online I've read "Audi service manuals say "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining tread depth."
 
Have a few sources at VW/Audi dealers (techs) I'm having do some digging today to see what technical guidance they can find.
 
This might be a good opportunity to get three more.

What I mean by that is, one was "free". so a new set would be buy 3 get one.

Use the others for summer spares or summer use?
 
This might be a good opportunity to get three more.

What I mean by that is, one was "free". so a new set would be buy 3 get one.

Use the others for summer spares or summer use?
Next year the other 3 will get replaced. Why buy 3 when it doesn't appear to be necessary?
 
Next year the other 3 will get replaced. Why buy 3 when it doesn't appear to be necessary?
I am not an expert on car tires, not much experience with little cars.

I was just thinking that 4 / 32 is low, real low, or at least it would be on a truck.

so next year you will be in the same boat, you will have one tire worn differently, and 3 new. Assming you have no damage, you will have to replace 1 before the other 3. To me it would be a PITA.

The other ones are IMO low in tread but not dead, they could be used as spares.
 
I am not an expert on car tires, not much experience with little cars.

I was just thinking that 4 / 32 is low, real low, or at least it would be on a truck.

so next year you will be in the same boat, you will have one tire worn differently, and 3 new. Assming you have no damage, you will have to replace 1 before the other 3. To me it would be a PITA.

The other ones are IMO low in tread but not dead, they could be used as spares.
I will only put 3-4k on these until March next year when I swap my summers back on...the new one may be at what...8/32 at most? Replace 3 next fall I'll be good to go. The fronts are well within the useable range at 4.5/32".
 
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I will only put 3-4k on these until March next year when I swap my summers back on...the new one may be at what...8/32 at most? Replace 3 next fall ill be good to go.
I am a stickler on tires. Maybe put the "old tire" in the position known to you to wear less per year, perhaps the rear? Many variables.
 
I am a stickler on tires. Maybe put the "old tire" in the position known to you to wear less per year, perhaps the rear? Many variables.
I am too - I ypically replace tires once at/below 4/32" but I'm not there yet. What do you mean "old tire"? 2 fronts are at 4.5/32". Rears are at 6/32" with the new one (subject of this post) that replaced the old one (which was also at 6/32") at 10. Always put the best tires in the rear - which is what I've done.
 
Thinking on this, I've done this on 2 of my other awd VWs sets of tires...but in both cases the other 3 tires were only a 1-2 32nds of an inch difference vs. the new one replacement one. Atlas (Haldex) got a new tire replaced as well as one of my summer tires on the Sportwagen, both free with the DT certs (less cost of cert!).
 
I am too - I ypically replace tires once at/below 4/32" but I'm not there yet. What do you mean "old tire"? 2 fronts are at 4.5/32". Rears are at 6/32" with the new one (subject of this post) that replaced the old one (which was also at 6/32") at 10. Always put the best tires in the rear - which is what I've done.
old tire was referring to your "new" tire" ........as when you go to replace the other 3, your "current new" tire will be the old one........older, not old, get it?
 
The joys of AWD.. I've bought one or two used tires to match the other good 2/3 on my 4x4 and AWD vehicles.

I know some tire shops in my area will refuse to sell you less than a full set for an AWD vehicle unless the tires are basically brand new.
 
The joys of AWD.. I've bought one or two used tires to match the other good 2/3 on my 4x4 and AWD vehicles.

I know some tire shops in my area will refuse to sell you less than a full set for an AWD vehicle unless the tires are basically brand new.
The joys of having extra wheels/tires so you only take the one that needs to be replaced so no questions.
 
old tire was referring to your "new" tire" ........as when you go to replace the other 3, your "current new" tire will be the old one........older, not old, get it?
Ah ok, got you. It won't wear enough in 3K miles to make much of a difference, will likely be 8-9/32" by spring when I remove it...it will go up front with 3 new tires in the other positions next fall when I replace them.
 
More logging...this time on the highway to get a bit more steady state/straight-line data. As expected, the average of the fronts (smaller @4.5/32") is higher than the average of the rears ([email protected]"/1 new @10/32"). I see no difference in the awd behavior due to this slight difference when logging it. It's basicaly "not on" when cruising. New tire is in the right rear position and you can see that comparing left to right for the rear. I also find the left front seems more erradict than the right...I do get an occasional code for "low mag field" on the left...replaced the speed sensor and still the same...has to be the hub.

 
1.3% will never bother a thing. Vehicles are made to tolerate such small variences with ease.
Heck even Toyota intentionally made the rear axle 0.7% taller on the GR Corolla and uses the center diff clutch pack to make it all work on the road with the idea being that it will more easily rear bias at hard throttle for handling.

Honestly with VW I'd be more worried about the tire pressure system getting mad just because it counts tire rotation, though setting pressures and then resetting the system from the radio menu should more than account for that. Slick surfaces would trigger the tire pressure warning for me in the GTI when it would have random slips I didn't detect and it would see consistent small wheel spin at low speeds.

The side to side difference would be best to deal with on the front since the AWD VWs retain an open diff on the front and I believe that the rear uses a clutch pack to engage it. I'm more familiar with the new one in the MK8 R so I'm not too well versed on the rear setup on the earlier cars, but they've all retained the open front and use the brakes to simulate limited slip.
 
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