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I’ve read about and experienced the dilemma of having tires possibly exceeding the circumference difference maximum recommended by Subaru. It occurred to me that using the probably never-used spare in the spare tire compartment could be problematic If your regular tires have significant wear. Hopefully it’s not a problem if you’re in civilization and can remove the spare after a few miles. What about if you’re not and have to drive more than a few miles. Anybody have an idea (ballpark guess ) on how many miles, with the maximum wear difference is too many? I realize there is a huge number of variables involved so just curious about any comments. Thanks
 

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2015 Outback 2.5i Limited, Ice Silver/Black
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2,161 Posts
I wouldn't really sweat it. You're getting the tire repaired/replaced ASAP anyway.
 
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Just realize that every time you take a tight turn, the outside wheels are turning more than the inside wheels and the front wheels are turning more than the rear wheels. Our cars have open differentials side to side and some kind of electronic clutch front to back.

Having said that, if you drive for a long time or at high speed with a wrong size tire, it can cause problems mainly for the center differential.

This is why I think it's a mistake to get larger diameter tires without a matching spare. In the 2020 Outback the space saver spare is supposed to have a matching diameter, but because of wear, the chances of having an exact match between a spare and worn tires will never be achieved. That small amount of difference shouldn't be catastrophic as long as you have the tire changed/fixed ASAP. I don't think there's a set number of miles that someone can come up with that is ok/not ok but in my mind less than 50 miles is almost certainly ok as long as the mismatch isn't too great, otherwise we'd have reports of constant center differential failures every time someone uses a spare tire.
 

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2018 3.6r Touring, Black, modifications: 255-55-zr18 Continental DWS06
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The manual indicates, if you have a front flat, pull one rear forward and put temp spare on the back.

I had the temp spare on the rear for approximately 25-30 miles highway speeds 55+mph. That was more than 15k miles ago and no adverse effects.

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2019, Subaru Outback Limited 3.6R
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An uneven tire circumference would still beat walking in my book so...
 

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Donut spares are limited in speed and range. We needed to replace a tire in the middle of a trip, it wasn't a perfect match but got us the 2,000 miles home. Then we replaced it with a tire that was closer in diameter to the other three. No ill effects noted at all, but obviously don't want to do that forever. That extra tire with 2,000 miles on it is now our full size spare on our 20.
 
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