I replaced one tire on my awd car

[Jetronic]

I know b/c I understand the sytem and have "real-world" experience driving around on it for the last 6 years/112K miles and at times with tires/wheels that haven't always been the same circumference (but admittedly not wildly different). My take is simply that the fwd-based part-time awd systems that uses clutches to engage the rear wheels (and open diffs) don't have as rigid of a requirement for tire wear differences than are let on by tire outlets but I do get it, easy to just say "change all 4" than to have any chance of a come-back with awd system damage, even if it's completely unrelated/coincidence which I'd say is more often the case than damage from a few mm of tire difference which can happen with just air pressure differences.

I'm pretty sure air pressure differences were what led to failure in the cases where it wasn't tyre wear

I've driven those cars on snow and ice, with the factory supplied Continental SportContact in 245/45R19. Those tyres have very little grip on that surface. In a tight 90° bend (like a small intersection) the rear drive would kick in and out 4-5 times. understeer/oversteer/understeer/oversteer/understeer/oversteer. Unless I put the car in awd lock.

The system was just too agressive, at least in the version we got in Europe and I'm not saying the software is the same all over the world.

 

[TiGeo]

I'm pretty sure air pressure differences were what led to failure in the cases where it wasn't tyre wear

It would have to be egregious.

 

[Jetronic]

It would have to be egregious.

I added some to the previous post.

 

[Al]

The only thing I would say, although this is probably pedagogical...If you are talking mismatched sized, two of the same tread/size should never be on the same axle.