Subaru Outback: Studied Replacements

Q--A tire on my Subaru Outback was punctured and couldn’t be repaired. Our local independent tire dealer said the same tire is no longer available and recommended a Bridgestone alternative. That raises another problem: A different tire could trigger or trick Subaru’s AWD system and potentially cause damage. This was raised by a Subaru customer service rep, who suggested replacing all four tires. No thanks, because the other three tires have 9/32 inches of an original 11/32 inches of tread depth left. In addition, our lease on the Outback is up at the end of the month, and I would be giving Subaru four nearly new tires with no compensation. I wanted to get a tire that was as close as possible to the original-equipment tire, and I didn't trust the recommendation of the tire dealer.

I found detailed specs on Firestone’s Web site for current Firestone and Bridgestone tires but not the ones on our Subaru. A Firestone technical service rep was able to dig up the specs for the original-equipment Wilderness tires. When I asked him to recommend a replacement, he said the most important considerations were the overall tire diameter and the tire rpm — which sounds plausible. The closest match is the Bridgestone Affinity LH30 because it has the same overall diameter (26.6 inches) and RPM (783), and the same speed rating (97H). I hope he is right, because that is what went on the car, even though the tread is half an inch wider on the new one. Is there a better solution? What are the potential pitfalls with AWD or any drive system?

A—You certainly did your homework and it paid off! The most important thing is the overall wheel-and-tire diameter. If one wheel spins faster or slower, the all-wheel-drive viscous coupling will constantly compensate, overheat, and fail. Mismatched tires can also render the anti-lock brake system inoperative.

The fact that your replacement tire is a smidgen wider has little impact on the AWD or ABS.

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