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Mistmatching tires and AWD

I have a 2005 Subaru Outback with 2-3 year old tires. I recently got a puncture in a sidewall, and of course they don’t make a tire with the exact same tread pattern anymore. I took it in to the shop where I purchased the tires to get a replacement (they are warrantied - only 50 bucks to replace them). They said that putting on a tire with a different tread would ruin the AWD system. This is NOT a Subaru dealer, and I sort of doubt how much he really knows about their AWD system. I trust this shop, and they have had the opportunity to pull one over on me before and they didn’t. Thoughts?

BYLRPhil…

Yes, they were right…in a way. Or perhaps you misunderstood.
It is the difference in the diameters of the tires that is bad for your Subbie’s AWD, and NOT the tread pattern.

It’s not necessarily the difference in tread that is problematic, but rather it is the difference in the circumference of the new tire vs the other three tires. A difference in circumference that is more than “slight” can ruin the car’s viscous coupler that splits torque between the front and rear wheels.

In a situation like this, you have three options, as I see it:

  1. Buy 4 new, matched tires–obviously expensive.
  2. Buy one new tire and have it “shaved” so that its circumference matches that of the other three old tires. Not every tire dealer can do this, so you may have to shop around for a shop that can do this.
  3. Put on the new, mismatched/unshaved tire and wind up replacing the viscous coupler in a few months, to the tune of…maybe $600. Ultimately, this is probably the most expensive option.

I would recommend option #2, and I would also suggest reading your Owner’s Manual, which discusses the topic of mismatched tires.

I would be more comfortable with your question if it was phrased as such "one of the tires used on my Subaru has a circumference of yyy this differs by zzz from the other 3, is this difference harmful to my Subaru’s AWD system? My reply is that there is tolerance is everything mechanical, now just how much with you Subaru’s AWD is the question.

I take the advice as is, because I agree that different tread patterns can stress an awd system. How much before there is damage may not be realized in your life time with the car.
So the question as Dirty Harry would paraphrase “do you feel lucky, well, do you ?”

I find it hard to believe that a minor difference in the circumference of a tire can destroy the AWD coupler in a Subaru. It seems illogical. If you drive your car on curving, winding roads and maneuver it around in city traffic, all four wheels are never rotating at the same speed. So why should a tire rotating at a slightly different speed make such a drastic difference?

Regarding not finding the same exact tire as a replacement, I have found that discontinued tires are often available inexpensively on the Internet, through ebay.

From a web site for subaru tires…believe it or not …

“Because full time all-wheel-drive systems read traction from the tires, mis-matched tires are a bad.
This means all tires need the same tread pattern and size to within 1/4” circumference?. The same tread pattern means they need to be from the same manufacturer. If you have a non-repairable blow-out and the tires aren’t brand new you should replace all 4 unless there’s less than 1/4" circumference difference."

The AWD with a mismatched tire puts the system into more slippage(heat) constantly. This is especially true when turning sharply where you will make the system want to slip beyond design capability due to mismatched tire.

My mechanic(25yr Subaru master tech) says 1/2" difference is okay. 1" difference in cirucmference breaks internals of transmission.