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Brucey
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This handy website says the difference in diameter (and, thus, circumference) is 6%.




Nominal diameter of a 225/65R17 tire is 28.5" according to the tire size website. As someone noted, 11/32" tread wear (1/4" + 3/32") reduces the radius, not diameter, by that amount, from 14 1/4" (14.25") to 13 29/32" (13.90625"), about 2.4%.



See above.

The spare is intended for temporary use at limited speed for a limited distance. Long distances and high speed with mismatched tires is different, even if the mismatch is much less.
I knew someone on this site would find something wrong with my math. Looks like I halved everything. Not sure why.

Still. I get the spare is intended for temporary use and that's all I plan to use mine for.

Do you see any potential issue with using a full size matching spare but only doing a 4 tire rotation?

At no point in the tires life would it exceed factory differences according to my wrong but still applicable math.

The plan is just to use it to limp it to a shop assuming I can't patch on site.
 

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The spare is intended for temporary use at limited speed for a limited distance. Long distances and high speed with mismatched tires is different, even if the mismatch is much less.
You beat me to this. In the case of the spare the limited distance and speed is key...

For the OP - considering tire industry standard (as indicated by the Discount Tire folks) is to replace all 4 unless indicated otherwise in the manual (it isn't in the case of a Subaru), the insurance company should follow that. For the sake of longevity of major components in your vehicle all 4 tires MUST MATCH in size, make, model, and even speed rating. If the insurance company can't get one to match the other 3, they should cover all 4. Even if they didn't cover 4, both tires on an axle should match, for the most predictable handling.
 

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I knew someone on this site would find something wrong with my math. Looks like I halved everything. Not sure why.

Still. I get the spare is intended for temporary use and that's all I plan to use mine for.

Do you see any potential issue with using a full size matching spare but only doing a 4 tire rotation?

At no point in the tires life would it exceed factory differences according to my wrong but still applicable math.

The plan is just to use it to limp it to a shop assuming I can't patch on site.
As long as you kept your speed down and distance to the least necessary, I suspect you'd be much better off with a full (but slightly different) size spare than the compact spare.
 

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Brucey
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As long as you kept your speed down and distance to the least necessary, I suspect you'd be much better off with a full (but slightly different) size spare than the compact spare.
Well I'm glad I'm now not needing to figure out a way to do a 5 tire rotation with TPMS.

Now if only there was a way to fit it in the car. :laugh:
 

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I was in a collision where the other vehicle damaged the sidewall of a single tire beyond repair. However, their insurnace estimate only wants to replace one tire. They claimed Subaru recommends but does not require the same tire pattern and wear/diameter. Of course none of the tire shops will only do a single tire on a suabru for that reason. Has anyone has success convincing insurance otherwise?

The current tires are a matching set. They have 30k miles and are rated for 80k so the diameters definitely won't match up.
Tell them your neck hurts and you might have whiplash. ;)
 

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Not likely, but if the AWD warning light comes on after replacing a single tire then you may have a stronger case for the remaining tires.


"AWD warning light (AT)
This warning light illuminates if the vehicle is driven with front and rear tire sizes different from each other.
On 4AT models, this indicator illuminates when the drive mode is changed from AWD to FWD (with the fuse installed in the FWD switch)."
 

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This handy website says the difference in diameter (and, thus, circumference) is 6%.




Nominal diameter of a 225/65R17 tire is 28.5" according to the tire size website. As someone noted, 11/32" tread wear (1/4" + 3/32") reduces the radius, not diameter, by that amount, from 14 1/4" (14.25") to 13 29/32" (13.90625"), about 2.4%.



See above.

The spare is intended for temporary use at limited speed for a limited distance. Long distances and high speed with mismatched tires is different, even if the mismatch is much less.
I was going to say this.
If you have different sized tires the differential is slipping or absorbing energy. Temperature is bad for differentials just like brakes. To a certain temp you are ok, then you just not.

This of the spare tire as slamming on the brakes. Alot of abuse of a short period of time. yes there is wear, yes the temp goes up but because its so short, its OK. This is engineered in to the car. But having a different tires is like driving around on the free way with your emergency brake on even just a little. Real damage wont be done in a short distance but over many 10s or 100s of miles It can cause damage from both wear and or over heating.
 

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Everything I've read says all tires must be replaced at the same time and routinely rotated to ensure even wear of all tires. Which made me wonder, what if I have a flat? Can I use my spare? On older models, you can remove a fuse which converts to front wheel drive but to my knowledge, the Gen 3 on up doesn't have that option.

You'll probably need to get some verification from a reliable source on replacement of the other tires.

If you have a flat, you put the spare on a rear wheel and the other three good tires on the other wheels (if the flat is in the rear, straight swap. If you have a front flat, spare at rear and that rear tire goes to the front).



Current gens (3+) have an electronic feature that detects the mismatch tire at the rear and deals with the difference in circumference (stops sending power there/coasts), sort of like the fuse idea.


In the OPs case, either buy all new (pay for 3) and sell the three used ones or shave it down. Only two options unless you want to trash the drivetrain.
 

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Current gens (3+) have an electronic feature that detects the mismatch tire at the rear and deals with the difference in circumference (stops sending power there/coasts), sort of like the fuse idea.
There is no physical way for subaru drivetrain to "stop sending power" to an individual wheel. However, if a mismatch is detected (using wheelspeed sensors) the AWD system can disable itself and light some indicators on the dash.

NOTE: When I say "disable itself", I mean that the center diff will not attempt to 'lock up' and the ABS system will be disabled too. (because the ABS system is used to direct torque AWAY FROM individual wheels)
 

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Do you see any potential issue with using a full size matching spare but only doing a 4 tire rotation?
Yes I see potential issues - read your statement then read the owners manual, you might be out of Subaru's specifications.

Could you rotate all 5 tires until the future spare gets just low enough that it passes specifications for replacing a new tire, and thereby gains your lower end range if it also replaces a tire with less tread? Let's say you have 2/32 to play with - an 11/32 new tire can only cover 9/32-11/32. If you have a 9/32 spare you double your coverage to 11/32 to 7/32, covering most situations.

Personally, I wouldn't worry about it. There's a reasonable chance you'll be in spec, VTD's are beasts, and there's no compelling trend. Over 20 years I have seen enough "bought new/rotated frequently/always been in spec" people get torque bind that it doesn't seem to be a compelling trend of issues if we ignore the eggregiously ill maintained stuff which clearly yours won't be.

Why not rotate all 5 and get your interior all trashy from these gross winter roads? ! haha
After a few years the spare becomes dated and a liability with any significant run time. If significant run time and 7 years of use aren't parts of the equation then maybe the original question isn't that critical?
 

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Brucey
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Yes I see potential issues - read your statement then read the owners manual, you might be out of Subaru's specifications.

Could you rotate all 5 tires until the future spare gets just low enough that it passes specifications for replacing a new tire, and thereby gains your lower end range if it also replaces a tire with less tread? Let's say you have 2/32 to play with - an 11/32 new tire can only cover 9/32-11/32. If you have a 9/32 spare you double your coverage to 11/32 to 7/32, covering most situations.

Personally, I wouldn't worry about it. There's a reasonable chance you'll be in spec, VTD's are beasts, and there's no compelling trend. Over 20 years I have seen enough "bought new/rotated frequently/always been in spec" people get torque bind that it doesn't seem to be a compelling trend of issues if we ignore the eggregiously ill maintained stuff which clearly yours won't be.

Why not rotate all 5 and get your interior all trashy from these gross winter roads? ! haha
After a few years the spare becomes dated and a liability with any significant run time. If significant run time and 7 years of use aren't parts of the equation then maybe the original question isn't that critical?
The only reason I'm against a 5 tire rotation is because of Subaru's awful implemention of TPMS requiring a trip to the garage to turn off a dash light.

I think the 5 tire rotation is the best method if one ignores the dash light but I just hate seeing it.

There is always the electric tape method.

I agree about having more flexibility in the tread by having some miles on the spare though.

Guess I could run the current tires down then switch them out with the spare.
 

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If your/their insurance refuses to pay fort the replacement of all 4 tires, sue the perp who hit you in Small Claims Court.
 

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The only reason I'm against a 5 tire rotation is because of Subaru's awful implemention of TPMS requiring a trip to the garage to turn off a dash light.

I think the 5 tire rotation is the best method if one ignores the dash light but I just hate seeing it.

There is always the electric tape method.

I agree about having more flexibility in the tread by having some miles on the spare though.

Guess I could run the current tires down then switch them out with the spare.
I was joking about the “I see issues” comment BTW.

Oh snap the TPMS, right, I totally forgot those pesky pests!
 

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If your/their insurance refuses to pay fort the replacement of all 4 tires, sue the perp who hit you in Small Claims Court.
if it wasn’t their fault there’s no need for small claims, waste of time. You can get them to basically do whatever you want. They will bend over backwards to be fair and make it a “good” experience.

I’m assuming there’s no fault if they’re not accommodating here which can make it a little trickier depending on variables. One may not always want to play hardball with their own insurance.
 
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