Seemingly-obvious tire matching question

[jonathan909]

As a well-understood matter of course, we run "matching" tires on all four wheels in order to eliminate unnecessary strain on the drivetrain.

But how matching, exactly, does "matching" mean?

Case in point is my '01 H6 OBW, which recently blew a tire on the highway.  I think it was the delayed product of the long and heavily-loaded road trip last summer involving a lot of old computer and test equipment - and a sailboat - which featured a broken right rear spring (it had lost part of the first turn, which lowered it about an inch).  I think that the resulting lowered suspension over that wheel caused the tread to get scraped a few times, leading to the blowout about 8 months later.

Anyway, that's one down out of an otherwise-good set of four.  When this happened, we bought a new set of four, and now I'm staring at three perfectly good tires on rims that I think it would be stupid not to use.

Now, I stick with stock sizes, so in this case everything is 225/60R16, generally M+S.

The question is, then:  Assuming all of the tires are of the same rated size, what latitude do we have in mixing manufacturers?    Since I have three good tires on rims from my previous set, can I use, for example, two of those on the front, and two of the same size from a different manufacturer on the rear?  Is there enough real-world deviation in running geometry between manufacturers for this to be a problem?

Inquiring minds want to know!

Edited June 7 by jonathan909

[ocei77]

AFAIK. There is a diameter std of being within 5/32 circumference of the tires.

Mixing of tire tread patterns will have some effect on tracking, but again AFAIK  not detrimental to the differential.

I am sure someone will chime in, but I pray it is with empirical data. Not heresay.

O.

[lmdew]

Within 10% of dia.

 

[idosubaru]

Manufacturer or pattern difference doesn’t matter.  It’s size dependent. It’s like driving a Subaru or truck on pavement with locked 4WD, which binds when turning. Different size tires can cause the same exact issue by virtue of tire circumference difference rather than turning radius difference.

Subaru lists their recommendations  looks like others have posted that  

If it’s a VDC or has a VTD, they’re indestructible in regards to the rear transfer gear set.  Run 4 dramatically different tires and you won’t blow that thing up. Im joking but I don’t care to run a mismatched tire on VTD, it doesn’t matter.

From a physics stand point running one new tire on a Subaru isn’t a big deal if you pay attention to your diffs. But that’s too technical for public consumption.

On MPT transmissions with open front and rear diffs, If buying one new tire - place it where your tires wear the most (front or back). Rotate the older three tires until the new one wears down to the same tread depth and then they’re all the same.

The smaller tire diameters will be the “driven” tires since power goes to the one needing less torque. From an engineering stand point the larger tire won’t be the one the differential “sees”.

There are tire shops that will install two new tires - one on the front and one on the rear opposite side of Subarus for these same reasons. I never figured out why they do opposite sides except maybe just for traction on both sides of the car for rain/snow.

if you have rear vlsd they usually fail and operate as an open anyway. Lol. But if one cares, put the larger one up front which is usually seeing more wear on older Subarus. If it doesn’t then you’ll have the unfortunate situation of the smaller tires wear outpacing the new larger one.

Now if the three wear down you’ll be be stuck with one smaller tire and potentially 3 new tires which is a trickier situation to juggle regarding “mismatched tires”. 

If you have more than 4 tires just rotate them accordingly until they all match.  

[jonathan909]

All of which is extremely informative, but doesn't actually answer the question I asked.  Which was:

Does anyone here have a problem with running tires from two different manufacturers, (probably) paired as a "front set" and a "back set", assuming (of course) that all four are of the correct type and approximate characteristics (e.g. M+S)?

 

[forester2002s]

Aside from the same-diameter situation, I would expect that (or at least not-be-surprised if) on-highway driving performance would be affected.

I'm thinking that different manufacturers would have their own preferred tread-design and configuration of internal metal/fabric plies.

Just how any of this might affect emergency swerving or braking, or cold-weather grip, is an open question. Personally, I wouldn't risk it.

[idosubaru]

13 hours ago, jonathan909 said:

but doesn't actually answer the question I asked.  Which was:

Does anyone here have a problem with running tires from two different manufacturers

In anticipation of your typical responses like this I intentionally put “manufacturer” as the first word of my reply. 

23 hours ago, idosubaru said:

Manufacturer or pattern difference doesn’t matter.  

Manufacturer and type have no bearing in this context. 

[Numbchux]

I have never seen a printed thing from Subaru saying what is and is not acceptable.

But if the circumference of the tire differs even a little bit, it can change the rpms of the hub fairly substantially. And since circumference is effected 6.28x the tread depth difference, it's pretty important. And since it is exponential, technically a tread depth variation is not accurate, as 1/32 of variance on a 195/75r13 will have a much larger affect than on a 245/50r20.

2 different model tires, made to the same "size" can have a decent variation in circumference, so bad juju to mix and match. Even within the same manufacturer.

 

I've worked at 2 Subaru dealerships over the years, the first would shoot for 2/32, and the current one within 1/32. I will be shaving a brand new tire this afternoon for a customer who had a flat.

 

 

That is all the official answer. That is correct practice to ensure zero extra wear on your differentials. Anything more than that WILL cause wear, but it's incremental. And will depend heavily on AWD type (01 H6 could be MPT or VTD) and typical usage. Up to you to decide if you're willing to accept a bit. A small difference might make your transfer clutches wear out a few thousand miles earlier than they would have anyway.

If you buy a Forester MT that's been lowered and has Impreza-sized tires on it, and get a flat and put the Forester spare on it. You will make it about 45 miles before the rear diff (even though it's not an LSD) catches fire. Ask me how I know.....