Why do I have to replace ALL 4 tires for just one flat?
Because the ONE new tire is a little taller than the others even if its the same size tire. That “taller” part wears the all-wheel-drive in a very expensive manner. It is a Subaru issue more than other cars.
It depends on the difference in diameter between the new and old tires. Some large tire shops have tire shaving equipment that can match the new tire diameter to the others.
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Who in their right mind would buy a vehicle where if you get a flat tire,all four tires have to be replaced?
NO!
Tester
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It’s only 1 yr old? 10k miles? Go to used tire store and find a nice used tire.
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measure the thread depth on the tires and compare that to the specification for the new tire. If the difference is 2/32 or less, you are fine. If it is > 2/32, then you are inviting problems, and should either shave the new tire or replace all 4.
If you have more than about 20k miles on the tires, you probably fail the above test.
Subaru actually specs zero difference, which is impossible to achieve. The 2/32 number comes from other sources.
It’s a simple matter of the OP asking himself the following question:
Do I want to risk damage to the vehicle’s center viscous coupler and (possibly) to the transmission, neither of which would be covered by the warranty?
Why would that type of damage not be covered by the warranty?
Because operating the vehicle with non-matching tires will be considered to constitute Owner Negligence, and damage that is caused by Owner Negligence is never covered by warranty.
So, the idea of buying a used tire should be immediately discarded–unless that tire is of the same brand, size, and tread depth as the other 3 tires. The chance of finding that is… remote.
Probably the best bet economically would be to buy one new tire of the same brand and size as the original tire–from a tire store than can “shave” the tread so that it matches the depth of the other 3 tires. Not that many tire stores can or will do this, but they do exist.
Why?
There are places where you can get a matching tire model, like this one:
http://stores.ebay.com/Bestusedtires?_trksid=p2047675.l2563
My wife’s relatively new car (~20K miles at the moment) trashed one tire by hitting a curb, I was able to get a replacement of the same model and even matching wear almost perfectly, had it delivered in 2 days for free.
Subaru specifies that the maximum allowable deviation between the circumferences of the tires is 1/4". That is in the owners manual. 1/4" is 8/32". Divide 8 by pi (3.1416) and you get somewhere between 2 and 3. Since most tire tread depth gauges measure in 1/32" increments, 2/32" is a safe figure to use.
I believe your OEM tires are 10/32" new so for about $3, you can get a manual tread depth gauge and if all your current tires are 8/32" or higher, you can get one new tire.
This is the allowable difference in diameter, so the allowable difference in radius (tread depth) would be half this.
The allowable difference is circumference, not diameter.
I’m talking about the “between 2 and 3” should be the allowable difference in diameter, right?
Keith
Thank you for your detailed and thoughtful response.
I’ve got it
John
where did you find this spec, it’s not in my manual, that I can see.
That is diameter change. Divide by two to get max change in radius, which is what a depth gauge measures. That comes to slightly over 1/32.
Thank you, Bill, this was very helpful.
John N
Your right, my bad.
which is very difficult to measure with any accuracy. Essentially Subaru is saying they have to match exactly, within the measurement error.
I guess that viscous coupler technology they use is a way to provide affordable all wheel drive, but seems pretty sensitive to tire diameter. An ordinary pinion/ring differential isn’t as sensitive to tire diameter apparently. At least I’ve never worried about 1-2/32 tread mismatches, and never had a differential problem.
You said you never had a problem. How many miles have you driven with tires that did not meet that 2/32" spec?
Estimate if you can
Nothing much I can do except be sure I rotate the tires on a regular basis, and check frequently. Last time I checked, they were all at 8/32 as far as I could tell.