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All wheel drive tire replacement

Further to my earlier question, what is confusing me is the spare tire. It’s the smallish skinny space (and cost) saving donut tire. I am fairly certain it’s not the same circumference as the other tires but there is no comment as to hurting the differential etc in the owners manual. So what gives here?? I am certainly confused.

Doesn’t it, or the manual, say something like ‘not for use for more than 50 miles’?

This type of spare is for short period of time/milage only. Get to the next station that can repair your tire and not over 50MPH either.

For the short time like that, it will not cause any damage to the drive train.

That spare tire should have the words TEMPORAL molded into the sidewall.
As that word implies, that spare tire is for temporary use only.
This temporary use is brief enough so that it does not damage the center differential.

When AWD systems started to fail beyond what was expected, manufacturers, in order to hold down warranty claims (deny claims) started to look for reasons they could use to avoid paying for failed drivelines. Mis-matched tires was just one of the things they came up with. Tire stores quickly jumped on the bandwagon, seeing an opportunity to sell four tires instead of one…From that point, it became urban legend…

But I should tell you, I was forced to drive on a compact spare in a Crown Vic for 250 miles at 50 mph. A week later, the rear end (differential) failed. I should have taken the time to put that useless spare on the front so as to match the rear tires…You can use those space-saver spares for a few blocks, but that’s about it…

Many tire stores have tire truing machines that can be used to shave down a new tire so it exactly matches the other three…You are NOT compelled to by four new tires…

The only car makers I know of that actually care about AWD matching tires is Subaru and Audi. The reason is they truely use full-time AWD system. Both seeming to want the tires around 1/2" to each other in rolling circumference.

The truth is very few Subaru systems fail under warranty. They usually have the clutch pack fail in the automatic transmission which is also a wear item in the 100k-250k range. The manual transmission Subaru AWD suffer few failures.

Of the people I know with Audi never heard of a single drivetrain problem in their AWD.

I concur it has turned to a tire shop bonaza selling new matched tires when they see AWD.

“Many tire stores have tire truing machines that can be used to shave down a new tire so it exactly matches the other three…You are NOT compelled to by four new tires…”

Worth repeating.