Tire Question

Rick89GTS

New member
Has anyone had any differential issues by replacing just one or two tires on your AllTrac?
(Same size and speed rating, just different amount of wear or brand)

I'm asking because the tire shop guy said that a Subaru owner replaced one blown out tire instead of all four and it burnt out the differential due to different tracking, ride height and wear pattern. He did say it depends on the car but he swears this happened. Sounds like 100% horseshit to me but just thought I'd throw it our there.
I cannot imagine that a Subaru diff would be that sensitive and fragile, it was probably on the way out anyways.
 
I used to run tires on the back that were 1/2" taller than the front and had no adverse affects whatsoever. I put probably 2000km like that.
 

alltracman78

Active member
Every time you drive and you're not driving 100% straight and flat you're turning each side of the diff at different speeds, just like you would if the tires were different sizes. The more the curve (or evenness) the greater the difference in the differential.

I live (and grew up) near Boston Ma. There aren't many places in the world that have curvier, uneven roads ( as far as general driving goes). I've also been a mechanic and around cars for over 20 years. Problems with differentials on the average car are next to non existent.

Unless you have a locking diff (or to a lesser extent a clutch type diff) you shouldn't have any issues.
 

entrax

Member
wouldn't make a difference. i had different tires front/back (same size, but different brand, and definitely different tread), diff didn't have an issue. only thing you'd stress is the center viscous, but it IS viscous so it's not super strong to begin with.

changing one tire is even less stress. assuming it's for your 165, it's open diff for both front and rear. those two diffs would relieve any differences without even being an issue for the center viscous
 

underscore

Well-known member
It depends on the difference in wear and diameter. If you have the same make and model of tire with minimal difference in wear you should be fine. If they're different by too much the odds of things breaking goes up. Ie if this guy had 3 nearly bald tires and then put on one brand new tire or a different model that's a different actual diameter then he's more likely have a bad time. In a pinch I'd at least put the different tire on the front with the open diff rather than having an LSD constantly slipping. But considering the cost and difficulty involved in replacing the viscous coupler or either of the diffs I'd rather just pay a couple extra bucks and have 4 matching tires. The VC probably wouldn't grenade but the fluid may get cooked and not function properly which may not be easily detectable.

Our cars are at least a lot more robust than a Subaru, I know those things have a big fan base but they seem to blow up in all kinds of different ways pretty often, even with no abuse.

alltracman78":318izjrj said:
Every time you drive and you're not driving 100% straight and flat you're turning each side of the diff at different speeds, just like you would if the tires were different sizes. The more the curve (or evenness) the greater the difference in the differential.

The differential between each corner of the car will vary as you drive though. If you have one mismatched tire the differential is always the same way which is what would cause problems.
 

freddie

New member
What happens when you have to use your spare wheel and it is a space saver type? Somewhere in your car's
hand book is instructions on using such a wheel and guideline to speed and driving.

I don't think it is the diameter that matters but the torque resistance being transferred back into the diff.
Imagine one light skinny wheel on one side with a heavier more grippy wheel/tyre on the other side. In
normal driving probably nothing will happen, but doing launches at the traffic lights or burn outs, you are
looking for trouble. Why/ Because as you are turning one wheel has a greater distance to travel than the
other one so the diff has to cope with this as it has been designed to do. Now add the unequal torque or
or load due to different resistance (different wheels) ..................There could be a bang.
 

RedCelicaTRD

Moderator
Actually Subaru transmissions are that fragile. The sensitivity to mismatched tire size is just a side effect of the transmission design they use. Also happens to Audi from what I've read. The design that Toyota uses is more tolerant of the mismatch.
 

FC Zach

Active member
freddie":3q7zygnr said:
What happens when you have to use your spare wheel and it is a space saver type?

For our cars (in factory form), the spare is the same diameter. . for those that have aftermarket wheels, the spare should be the same diameter in the event it is needed.
 

FC Zach

Active member
underscore":3l6h9os5 said:
It depends on the difference in wear and diameter. If you have the same make and model of tire with minimal difference in wear you should be fine. If they're different by too much the odds of things breaking goes up. Ie if this guy had 3 nearly bald tires and then put on one brand new tire or a different model that's a different actual diameter then he's more likely have a bad time. In a pinch I'd at least put the different tire on the front with the open diff rather than having an LSD constantly slipping. But considering the cost and difficulty involved in replacing the viscous coupler or either of the diffs I'd rather just pay a couple extra bucks and have 4 matching tires. The VC probably wouldn't grenade but the fluid may get cooked and not function properly which may not be easily detectable.

The differential between each corner of the car will vary as you drive though. If you have one mismatched tire the differential is always the same way which is what would cause problems.

I agree!

Simple answer: You can but you shouldn't.
 

alltracman78

Active member
You all remind me of a guy trying to cut 1/16" off a 2 x 4 for a wall stud, worrying that if they're not all the same size the wall will be uneven.

underscore":m19wins2 said:
The differential between each corner of the car will vary as you drive though. If you have one mismatched tire the differential is always the same way which is what would cause problems.

Please explain how adding 1 mismatched tire cancels out the wheel speed differences caused by curves and uneven roads.

Going from a brand new tire to one that is completely worn out is just over a 2% difference in rolling diameter. That's about as extreme as you're going to get. And since it's pretty unlikely you're only going to replace 1 tire if the other 3 are completely worn out, your difference is probably going to be less.

And how long are you going to be running 1 tire different? The other tires are already worn, and once they're worn out, (I hope) you'll be replacing all 4.

Also consider that difference is going to the front or rear diff first, to be partially cancelled out before it even reaches the viscous coupling.

And if your tire pressures are not EXACTLY the same you will have different rolling diameters. And unless you're using a laboratory grade gauge that reads to .01 PSI it's pretty certain your pressures are going to be off some. Even if you check them all the time.

And if your side to side alignment isn't identical your wheels are going to be turning different speeds (your front and rear alignments are already different). How likely is it your alignment is that exact, even if you went to a "race shop" to get it done?

Coupled with all the other reasons, the likelyhood of all 4 of your tires (or even 2) turning the exact same speed at any given time is probably close to zero. It's fairly likely your tires will have a higher difference in rotating speed more than half of the time. And if your car can handle that for thousands and thousands of miles, I'm sure it will be ok with 1 tire being newer than the rest.


Am I telling you that you should put just 1 new tire on? No. Ideally you match at least 2.
But if your tires are halfway worn and you have 1 go bad, especially if your wallet is short, you'll be fine putting just 1 tire on.
 

underscore

Well-known member
If you are going to swap just one tire the main thing would be ensure it's the same actual outside diameter, ideally the exact same brand and model of tire. Just because two tires are listed as the same size doesn't mean the rolling diameter is actually the same. ie if you have 3 good Kuhmo's I wouldn't put on a Toyo, even if the sidewall says they're the same size.

alltracman78":2gqx09jp said:
Please explain how adding 1 mismatched tire cancels out the wheel speed differences caused by curves and uneven roads.

That's not at all what I'm saying. With matching tires, if you're on curves and uneven roads the difference changes with each bump and curve (a little bit one way, a little bit the other way, etc) so the drivetrain isn't constantly slipping in the same direction, unless you're driving in a circle for miles and miles but you're likely to get dizzy and throw up before wearing anything out. Now if you put on a mismatched tire and drive straight the slip is always going the same way and always happening, so heat is constantly being generated and wearing things out.
 
Mismatched tires - effect on front / rear differentials - no significant negative impact if they were independent.

Mismatched tires - effect on driveline (combination of front & rear differentials and viscous coupling/driveline linking front to back) - more significant.

Lift the rear of a Celica AWD up, leaving front wheels on the ground, transmission in neutral. Then try turning the propeller (or front-to-rear driveshaft) by hand. Feel all that resistance while you trying turning that driveshaft (and by hand, you can barely turn it at even the slowest speed)? What you are working against is relative speed difference between the front and back drivelines through the viscous coupling. That torque, and how it loads the gears/bearings, puts a heck of a lot of load and stress on the driveline over and above normal driving. Will it break something - likely not. But the added stress will result in accelerated wear over time. If you are rally racing your ST185, then mismatched tires would not "significantly" increase driveline stress (it is already under a tremendous amount of stress on those terrains). If you are wanting to preserve your ST185 and only street driving it, from a relative perspective, mismatched tires will be significant regarding driveline stress compared to what your ST185 would otherwise see. Over time, if not corrected, it will make a difference in driveline wear. One can decide if they feel this stress is significant or not to them if faced with the option to have mismatched tires on an ST185 or not.
 
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