Matching a replacement tire

[Stevo F]

My son reported to me that the nail in the tire on the '98 Legacy GT (automatic) is in the sidewall (not what a Subaru owner wants to hear). I also happen to have a tire that had been on our old Impreza Outback Sport (the same thing happened  with the dreaded nail in the sidewall) same size 205-55- R16 and the same tread depth (average of 7/32" for both current and extra tire). Both are all season radials, but are different brands.

How safe would it be to use this tire? I believe the tire shop can check the exact diameter of each tire for me. I certainly want to avoid a $1,000+ clutch pack repair, but would also like to avoid buying a $400 set of tires when these have another 30,000 miles left on them.

[FerGloyale]

Wire your FWD circuit (fuse holder) to a switch.

Then you can run the mismatch on the rear, in FWD most of the time.

Then when roads are wet and slippery, snowy, or gravel, flick the switch off and be in regular AWD

I've done this for my cars specifically so that I can run mismatches front to rear.

Convienient for when you get a flat too, no need to open hood and find a spare fuse to go to FWD.

[Stevo F]

8 minutes ago, FerGloyale said:

Wire your FWD circuit (fuse holder) to a switch.

Then you can run the mismatch on the rear, in FWD most of the time.

Then when roads are wet and slippery, snowy, or gravel, flick the switch off and be in regular AWD

I've done this for my cars specifically so that I can run mismatches front to rear.

Convienient for when you get a flat too, no need to open hood and find a spare fuse to go to FWD.

This brings up the question of whether disabling AWD using the fuse on a long term basis will do any other harm to the car. I’ve got another Subaru with some rear end noise that I would consider running in FWD much of the time as well.

[FerGloyale]

2 minutes ago, Stevo F said:

This brings up the question of whether disabling AWD using the fuse on a long term basis will do any other harm to the car. I’ve got another Subaru with some rear end noise that I would consider running in FWD much of the time as well.

In my experience, and from a mechanical perspective iIt does not damage anything in the drivetrain.

The one thing it "Could" do is affect the life of the Duty C solenoid, as FWD gives it a 12v+, 95% duty cycle square wave.  As opposed to a variable cycle ranging about 20-60% until slip is detected.  So it could maybe wear that out faster. 

I only charge $280 + fluids for Duty C replacement.  vs. $400+ for tires?  IDK, it's up to you, your gonna spend money somewhere, but the FWD switch will help you get by today.

For my money, I'd go FWD to run your mismatch for now, and just get 4 new tires on as soon as you can.

[idosubaru]

23 minutes ago, Stevo F said:

This brings up the question of whether disabling AWD using the fuse on a long term basis will do any other harm to the car. I’ve got another Subaru with some rear end noise that I would consider running in FWD much of the time as well.

It’s a nonissue. It’s basically an extension of ones personality.  

If you like conspiracy theories or hedging your bets on unlikely scenarios - yes there’s a 0.000000000000000003% chance it could cause some issues. In which case those people would need to also carry a helmet for escalators and elevators and go out of the way to avoid left hand turns while driving. 

The more practical type will learn and ask people that have actually done it for years across more subarus than they can remember , realize nothing at all will happens, and carry on. 

That being said the FWD fuse isn’t ideal to be used as a cloak or to limp along to mask known, undiagnosed, symptoms - that’s not a recipe for long term success. So if you’re going to do it - it’s ideal to know why you’re doing and that it’s appropriate.  It’s not a magic fuse. 

[Stevo F]

Thank you. Ideally I want to run the car as normal in AWD (especially since I’m not the primary driver and the car is normally 50 miles away). Getting to the original question- is using the same size, same wear tire going to cause issues in the long run? The tires have a lot of life left in them and hoping not to replace anytime soon. I’ve already put new tires on 3 of cars in the past 6 months so hoping to avoid a 4th. 

Not that I’d ever do this with any car, but the previous owner of the Impreza Outback Sport I just bought had 3 different types of tires, run presumably  for a long time, but all within 1.5/32” of tread with each other. I got new tires immediately, but not the least sign of torque bind on the car

[FerGloyale]

If they are very close in size and wear, it should be ok.

It's actually easier on an Automatic in any case, as the rear transfer is loosely driven under most circumstances anyhow.

Manuals it's worse for them.  The side faces of the spider gears aren't meant to spin constantly, which is what happens just going in a straight line down the road with mismatched tires through the true center diff.  

Got a tailor's measure?  Measure the cirumference of tires and check to see how close they are and then make the call?

[Stevo F]

24 minutes ago, FerGloyale said:

If they are very close in size and wear, it should be ok.

It's actually easier on an Automatic in any case, as the rear transfer is loosely driven under most circumstances anyhow.

Manuals it's worse for them.  The side faces of the spider gears aren't meant to spin constantly, which is what happens just going in a straight line down the road with mismatched tires through the true center diff.  

Got a tailor's measure?  Measure the cirumference of tires and check to see how close they are and then make the call?

Thank you. I’ll try the measuring tape to the circumference of each. What would be an acceptable  difference in circumference?

If we decide to put it on, do you recommend moving it to the back? (it’s on the passenger’s front right now).

[FerGloyale]

1 hour ago, Stevo F said:

Thank you. I’ll try the measuring tape to the circumference of each. What would be an acceptable  difference in circumference?

If we decide to put it on, do you recommend moving it to the back? (it’s on the passenger’s front right now).

 

Ideally, less than 1/16" rolling diameter.

Bare minimum, 1/4"

Yes put mismatch on the back, the open diff will handle the side to side variation and mostly won't be under load.

Always want matched on your steer tires anyhow even on manuals.

Edited July 22, 2019 by FerGloyale

[Stevo F]

I ended up taking masking tape and going around each tire in question, then laying it out and measuring. The circumference of tire with the puncture is exactly 78" . Then I measured the one I want to put on and it came out to 77 7/16"- 9/16" less which seems significant.

I then decided to see how close the other 3 tires on the car are. The right rear had 6.5/32" of tread, the ;least of all of the tires so i measured the circumference and it came out to 77 9/16", which is 7/16" less than the right front and only 1/8" larger than the one I want to put on. Given most of my cars vary 1/32" between tires anyway, this seems in line with that.

I'm thinking of moving the left rear to the right front and putting the replacement tire on the left rear so the two "smallest" tires would both be on the rear.

Actually, I'm also realizing that since the extra tire I want to put on is currently not mounted or inflated, that the circumference will increase when inflated- so the difference between the tires may be insignificant once it is inflated.

Edited July 22, 2019 by Stevo F

[Stevo F]

I'm going to just get a new set of tires since this is actually one of lower mileage, nicer Subaru's we have.

I can save the three good tires and match my extra tire with it for some other car down the road (maybe our 2005 Impreza which is more of a beater, but I just put new rubber on that already).

[idosubaru]

On 7/21/2019 at 9:54 PM, Stevo F said:

I ended up taking masking tape and going around each tire in question, then laying it out and measuring. The circumference of tire with the puncture is exactly 78" . Then I measured the one I want to put on and it came out to 77 7/16"- 9/16" less which seems significant.

I then decided to see how close the other 3 tires on the car are. The right rear had 6.5/32" of tread, the ;least of all of the tires so i measured the circumference and it came out to 77 9/16", which is 7/16" less than the right front and only 1/8" larger than the one I want to put on. Given most of my cars vary 1/32" between tires anyway, this seems in line with that.

I'm thinking of moving the left rear to the right front and putting the replacement tire on the left rear so the two "smallest" tires would both be on the rear.

Actually, I'm also realizing that since the extra tire I want to put on is currently not mounted or inflated, that the circumference will increase when inflated- so the difference between the tires may be insignificant once it is inflated.

Im seeing tires on the car off the car inflated not inflated punctured and tread depth and circumference. A bit convoluted to keep track of. 

Depending what combo you got going on just put the larger tires on the front and put a little more air in the rears if you want a little extra margin.  The fronts then wear down to match the rear. I’ve done it countless times on Subaru over the decades.   

Edited July 30, 2019 by idosubaru

[nelstomlinson]

If it's just a nail hole, jam a plug in it from outside and run it.  A cut sidewall would require a boot, and most tire shops use safety concerns as an excuse to sell you four new tires, but a nail hole doesn't compromise the structure and you can easily plug it from outside, no tire shop needed.  Or, just put some green slime in the tire and run it.  I've had fair luck with that stuff.