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OT - Slightly Mismatched Tires on AWD Car - Showstopper?

N

npowell

Member
Gentlemen,

I have a 2013 Subaru Forester. Three months ago, I gashed one tire on a curb (at speed) and replaced just that tire with the same size, but different brand of tire. The other three tires are original and have about ? of their useful life left.

It just came to my attention that the Owner?s Manual specifies all four tires should always be replaced simultaneously to avoid mismatched rotation speeds, which could damage the center differential (the one in the transmission/transfer case that accommodates speed differences between the front and the rear). Conceptually, I knew it could be an issue, but I really didn?t think that tread wear difference on the same size tires would be enough to cause a problem. Now, I am seeing things online that suggest otherwise.

Per the link at the bottom, Tire Rack indicates Subaru?s spec is within ? inch of tire circumference or 2/32 tread wear. On my car, as far as I can tell, the difference between the new tire and the originals is probably about twice that. The attached driver?s side pic shows the difference in rotation that accrued over approximately 150 feet of straight driving. By my math, it seems the rear probably does about 5 more rotations per mile than the front (which is around 1%). For reference, I?ve also attached a pic of the passenger side after the same 150 feet of driving; surprisingly well-matched.

I suppose I ought to suck it up and do something about it (either have the new one shaved or get four new tires on the car). Before I leap, I figured I might check around to see if anyone knows if the Spec/Owner?s Manual is a spook story to sell tires, or if that much mismatch really can damage the car.

Thanks,

Neil

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You need to get a tire that is as specified circumference to avoid internal damage. (I am a 5 subaru person and own 3 as we speak) the older systems were not too particular. the newer symmetrical drives will take a beating with that much variation. There are two solutions Find a used tire of the same circumference, or 4 new. It really does matter. Jim
 
According to my neighbor, a 35 year mechanic, yes it can in fact cause expensive damage. He has told my family members to never run an awd vehicle without an equal set of 4 tires, and that tire rotation is very important on an awd. I have never tried to prove him wrong, I just change out tires in sets of 4 on our awd suv.
 
From your pics, I can [i:ae7210027d]easily[/i:ae7210027d] see the difference between the new tire and the other 3.

From looking at the photos, I'd recommend repeating the test, but having someone video that side as well, then play back and visually count the rotations of each of the two wheels. I suspect the number of rotations of the rear wheel might surprise you.
 
The mismatch can and will damage the transfer case or the diff. My suggestion would be to put 4 new on and keep the new one incase you mess up one of the new ones
 
My sister just bought 4 new ones for her I think 2016 forester. 1 got gashed She has 3 of the originals left you could buy. She is in Columbus Ohio. If you are near there I can get the phone number.
I'd get yours straightened out and hope like heck it did not ruin anything yet. A fluid change in that unit may be in order too.
 
(quoted from post at 11:55:46 01/23/19) From your pics, I can [i:645984d393]easily[/i:645984d393] see the difference between the new tire and the other 3.

From looking at the photos, I'd recommend repeating the test, but having someone video that side as well, then play back and visually count the rotations of each of the two wheels. I suspect the number of rotations of the rear wheel might surprise you.
Click to expand...

I got out of the car every 30 feet or so to make sure the rears weren't "lapping" the fronts. They weren't. The difference in the photo is the total difference over the 150 distance.

Still, it sounds like everyone is pointing me in the same direction- resolve the issue.
 
On my K1500 Silverado I had near new set on the frt. and badly wore ones on the back that were all same size. When I put it in 4 whl. drive. I had to back up to get it out. So I replaced the worn pr. with near new set and now will come out of 4whl like it should.
 
I don't get where the other posters are saying 4 new tires. You just bought 1 new replacement. Wouldn't the correct number of additional new tires be 3? That being said, I wouldn't think twice about replacing the 3 originals. If they are down to 1/4 useful life, just do it. You got use of the best 3/4 of their ware.
 
2X with Barts Dad, at a minimum replace tires in pairs on one axle.

With only 1/4 treat left, three more tires to match the new one would be ideal for winter driving.

Is tire rotation more important on AWD than on FWD or 4WD?
 
As I understand it, tire rotation is more important on an "AWD" vehicle (meaning one that always sends power to all 4 wheels on dry pavement). The reason is that the differential in the transfer case has accommodate front/rear speed differences, which would occur if the tires get mismatched.

In a "4wd" vehicle, the transfer case is typically in neutral with power going only to two wheels when on dry pavement. On low-traction surfaces (where you would typically engage 4wd), the lack of traction alleviates binding in the drivetrain.

In a FWD vehicle, you are always in 2wd (obviously), so there's no binding issue ever.
 
We have our second Subaru Outback, the original tires on the first one only lasted 26,000 miles! they didn't look that bad, but they were cupped and noisy. The tire dealer said it was from not rotating them often enough, and possibly from misalignment. We got it aligned, put new tires on it and at 60,000 they were worn, but not cupped. We liked the car, so we traded it in on a new one and I intend to rotate these more often. I think the bottom line is any car with full time AWD is going to be hard on tires.
 
The system is very symmetrical and all 4 would need to "peel" Nope. Subarus will pull up hill with 3 wheels on ice. Jim
 
The only thing I will say is I believe different model tires of same brand will never be quite the same in diameter and the defference between different brands bight be quite a bit for same listed size. I know I have found that on tires of different makes or models the difference in size can be quite a bit. So I would at least stay with same model tire in that instance.
 
I had the same thing happen to me on my AWD Mercury Mariner. 13 days and 400 miles after putting 4 new tires on I cut one and had to replace it. Problem was that the tires I put on had been discontinued so the replacement was the new version of the tire. The tread is similar but not exact, but the revs per mile spec was the same. No more miles than mine had on them I didn't think it was a problem.
 
I would get the other three new tires and be done with it. I don't see why the inter-axle differential would be stressed any more than the axle differential with the mis- matched tires on it
 
At the very least buy one more like the new one, keep them both on the same axel and you will be ok.
Click to expand...

Nope, not OK. Having different sizes front to back puts excess stress on the center differential in an AWD drivetrain. Not sure about the Subaru, but the center diff on my AWD Mitsubishi is a viscous coupled limited slip. Having different sizes front to rear makes it constantly have to slip a bit. Generates excess heat and will kill the viscous coupler.
 
I went through a similar thing a few months ago. I had one tire that always caused the car (?15 Crosstrek) to pull hard to the right and it wore real funny. I couldn?t get another tire the same so I bought 4 new ones and got a decent amount out of selling the 3 good tires. Cheaper than fixing the car and gave me peace of mind. Sam
 
Tire Rack can grind a tire down to match the rest that is what our Subaru dealer told use. Had a 2015 Forrester, now have a 2019 ascent.
 
A Subaru system is limited slip in front rear and center. It also uses selective braking to equalize loads. it is one of the best at actually doing what makes the car move in really bad conditions. Jim
 
(quoted from post at 18:39:47 01/23/19)
At the very least buy one more like the new one, keep them both on the same axel and you will be ok.
Click to expand...

Nope, not OK. Having different sizes front to back puts excess stress on the center differential in an AWD drivetrain. Not sure about the Subaru, but the center diff on my AWD Mitsubishi is a viscous coupled limited slip. Having different sizes front to rear makes it constantly have to slip a bit. Generates excess heat and will kill the viscous coupler.
Click to expand...

As far as I know, my Subaru also has a viscous coupler in the center.
 

Inter-axle diff has a viscous coupling that could suffer. Front and rear diffs on '13 Forester are open, to my knowledge.
 

I believe you're right about the new ones. On my '13, I believe the front and rear diffs are open, with no active brake control to transfer torque side to side. Much less capable than the newer ones.
 

Meaning, I am leaning toward having a tire shaved by Tire Rack. Doesn't seem like it's very easy to find someone local that will do that:-(
 
I have a Dodge Colt AWD. I have 2 tires one size and 2 tires another size. I have one large tire and one small tire on each axle. That makes both axles the same speed. Been like that for many miles and no problems. I don't have limited slip axles.
 
i would follow your owners manual. if it says they all need to be close in diameter they need to all be close in diameter.

if the other tires only have 1/4 tread it doesn't make much sense "to me" to grind down a new tire to match them. I'm afraid you'll need to bite the bullet and get 3 more to match the new one. if you're up to it you can sell the other 3 but with 1/4 tread i wouldn't buy them and others might not either.

we've got a AWD Chrysler 300 and i've been pretty faithful about keeping them rotated. they're a goofy 19" diameter Michelin too so i'm sure they are high $ to replace.

good luck
 
Before anyone comes long and says, "My tractor has two different tires on it and nothing has happened to the differential!" and/or "That's what a differential is SUPPOSED to do! Why is it a problem?"

It's because of speed. Tractors have very large wheels and move at speeds generally under 15MPH. Cars have very small wheels by comparison and frequently travel at speeds exceeding 55MPH.

The differential in a car is turning many times faster than one in a tractor, and with two different size tires, those little spider gears get to whipping around. The wear is exponential.

Those little spider gears are only meant to make small corrections, not to be continuously spinning at a high rate of speed.
 
. Same size tires on the same axles is essential. A little variation front axle to rear axle can be tolerated . Eg new tires on rear axle and half worn tires on front axle is ok.
 
It is called tire truing andthe only place that ever did it quit at least 40 years ago and building has been gone for years. At one time they had 3 alignment racks in use all at same time. Think there were some legalities about it when they quit doing it.
 
One thing I was always told, and I did not see it mentioned, is with 4wd and I would suppose AWD, not only keep the tires the same size, but also the same tread pattern, as one tire with a different tread will have different traction and then also cause internal unneeded wear.
 
There are variations in actual measured size between tires of the same NOMINAL size, but different brands/styles.

A 255/55R16 Firestone will measure out differently from a 255/55R16 Goodyear for example.
 
(quoted from post at 11:59:42 01/23/19)
(quoted from post at 11:55:46 01/23/19) From your pics, I can [i:4a472de7a8]easily[/i:4a472de7a8] see the difference between the new tire and the other 3.

From looking at the photos, I'd recommend repeating the test, but having someone video that side as well, then play back and visually count the rotations of each of the two wheels. I suspect the number of rotations of the rear wheel might surprise you.
Click to expand...

I got out of the car every 30 feet or so to make sure the rears weren't "lapping" the fronts. They weren't. The difference in the photo is the total difference over the 150 distance.

Still, it sounds like everyone is pointing me in the same direction- resolve the issue.
Click to expand...

Look for a button on the console for "X-mode". Not sure if the 2013 has it or not. X-mode is selective limited slip differential. If you have one tire different size, it will be affected by the X-mode setting, so turn it off.

The viscous coupler is for manual trans only. If you have the manual trans, you have the viscous coupler, and it will be affected by different tire size. The coupler is designed to handle different radius distance as the car is turning corners, and has slight different in tire dia. The greater the delta, the greater the viscous fluid must work to manage. This creates heat, and eventually wear in the coupling.

I have a 2010 Forester auto, so no viscous coupler, and no X-mode. I also have different brand tire on the right rear due to a tire damage incident. I've driven at least 7000 miles on that tire combo, and no issues at all. HOWEVER, the newer Forester is not as forgiving.

Depends on the shop, but some tire places will buy your old tires if they have nice tread, and aren't too worn. They may give you a bit of a break because they can sell them as used. Or, you can take them with you and sell on C-list maybe.
 
(quoted from post at 11:50:11 01/24/19)
(quoted from post at 11:59:42 01/23/19)
(quoted from post at 11:55:46 01/23/19) From your pics, I can [i:beff43f5b4]easily[/i:beff43f5b4] see the difference between the new tire and the other 3.

From looking at the photos, I'd recommend repeating the test, but having someone video that side as well, then play back and visually count the rotations of each of the two wheels. I suspect the number of rotations of the rear wheel might surprise you.
Click to expand...

I got out of the car every 30 feet or so to make sure the rears weren't "lapping" the fronts. They weren't. The difference in the photo is the total difference over the 150 distance.

Still, it sounds like everyone is pointing me in the same direction- resolve the issue.
Click to expand...

Look for a button on the console for "X-mode". Not sure if the 2013 has it or not. X-mode is selective limited slip differential. If you have one tire different size, it will be affected by the X-mode setting, so turn it off.

The viscous coupler is for manual trans only. If you have the manual trans, you have the viscous coupler, and it will be affected by different tire size. The coupler is designed to handle different radius distance as the car is turning corners, and has slight different in tire dia. The greater the delta, the greater the viscous fluid must work to manage. This creates heat, and eventually wear in the coupling.

I have a 2010 Forester auto, so no viscous coupler, and no X-mode. I also have different brand tire on the right rear due to a tire damage incident. I've driven at least 7000 miles on that tire combo, and no issues at all. HOWEVER, the newer Forester is not as forgiving.

Depends on the shop, but some tire places will buy your old tires if they have nice tread, and aren't too worn. They may give you a bit of a break because they can sell them as used. Or, you can take them with you and sell on C-list maybe.
Click to expand...

No X mode on this one; it's too old. And, yes, I have the manual transmission and viscous coupler.

Have a shaved tire on order from Tire Rack. Will probably see if I can get a few bucks for the prior replacement tire. Reasons for not replacing the other three at this moment get a bit complicated.

Thanks to all for the help!
 
Got one and thoroughly agree.
This will damage driveline. That's what the sensor is telling us.
Got a Ford that will disable the 4x4 and ABS if the tires are different also. This will happen if the spare is used also. Get four.
 
wow this is getting as much action as a "best oil" thread ! and since I'm speaking about oil does your Subaru burn a lot of oil ? That is the year the class action suit on oil burning is for. They will replace your engine if it is bad enough. My sister had one that year and it burned a lot of oil. She traded it off to the Subaru dealer for a 3 year newer model with automatic tranny and they get better gas mileage then the manual tranny she had.
 
(quoted from post at 14:31:57 01/24/19) wow this is getting as much action as a "best oil" thread ! and since I'm speaking about oil does your Subaru burn a lot of oil ? That is the year the class action suit on oil burning is for. They will replace your engine if it is bad enough. My sister had one that year and it burned a lot of oil. She traded it off to the Subaru dealer for a 3 year newer model with automatic tranny and they get better gas mileage then the manual tranny she had.
Click to expand...

I think it burns about a quart every 3,000 miles. Not worth the hassle to me to tussle with Subaru over that. I just make sure it doesn't get low.
 
NOT SO !the only time the spider gears spin at a high rate of speed is when one wheel is stationary and the other one is spinning as when a person gets stuck n the mud. one size tire difference will never hurt the diff! because the spider gears are rotating slowly in the cage. I have seen where the pin in the spider gears has turned blue from spinning and heat, plus worn and loose. ALSO... if this was such a big deal the company's would not be giving you a wheel barrow tire as a spare on a new vehicle!
 
(quoted from post at 19:58:36 01/24/19) NOT SO !the only time the spider gears spin at a high rate of speed is when one wheel is stationary and the other one is spinning as when a person gets stuck n the mud. one size tire difference will never hurt the diff! because the spider gears are rotating slowly in the cage. I have seen where the pin in the spider gears has turned blue from spinning and heat, plus worn and loose. ALSO... if this was such a big deal the company's would not be giving you a wheel barrow tire as a spare on a new vehicle!
Click to expand...

The wheelbarrow spare is an interesting subject, too. Unbeknownst to me (prior to a few days ago), the Owner's Manual specifies that you cannot use that wheelbarrow spare on the front. If you get a flat on the front, you have to use the spare to replace a rear tire, followed by putting the rear tire on the front. That said, in my ignorance, I drove a few miles with the spare on the front without catastrophic failure.
 
(quoted from post at 22:57:32 01/23/19) We have our second Subaru Outback, the original tires on the first one only lasted 26,000 miles! they didn't look that bad, but they were cupped and noisy. The tire dealer said it was from not rotating them often enough, and possibly from misalignment. We got it aligned, put new tires on it and at 60,000 they were worn, but not cupped. We liked the car, so we traded it in on a new one and I intend to rotate these more often. I think the bottom line is any car with full time AWD is going to be hard on tires.
Click to expand...

My experience is very different. My 2010 Forester ran the original tires (Yokahama Geolanders) to 160,000 miles. Minimal rotation. The Symetrical 4WD is probably the most advanced of its kind so you won't get "scrubbing" between front and back. The Forester is something like 90% power to the front and 10 to the rear, it moves power so smoothly you don't feel any difference. The tire brand and type is important too, the Geolanders are very good, but ride hard vs. other Yokahamas.

To the original topic, you do need to replace all 4 tires at once. The o.p. might be able to get 3 if the current one new one does't have too much use yet.
 
(quoted from post at 15:34:07 01/24/19) . Same size tires on the same axles is essential. A little variation front axle to rear axle can be tolerated . Eg new tires on rear axle and half worn tires on front axle is ok.
Click to expand...

Nope. Unless you don't plan on keeping the car and passing the damage you do on to the next owner.
 
......ALSO... if this was such a big deal the company's would not be giving you a wheel barrow tire as a spare on a new vehicle!
Click to expand...

It's been ages since I read the owners manual, but IIRC it emphasizes a maximum speed of 45 and no more than 50 miles when using the spare on my AWD Mitsubishi. There is also warning stickers on the driver doorjamb for tow truck drivers to let them know about the viscous coupled limited slip diffs.
 
Jim,
If I am correct, and you also Jim, this motor vehicle relies on all four tires being the same; so tire size on both axles must be equivalent, not just front matching and rear matching.
 
I know that sme information was in the owners manual for either my Dodge Caravan 2 wheel drive or the Olds Ciera also 2 wheel drive.
 
(quoted from post at 20:05:51 01/24/19)
(quoted from post at 19:58:36 01/24/19) NOT SO !the only time the spider gears spin at a high rate of speed is when one wheel is stationary and the other one is spinning as when a person gets stuck n the mud. one size tire difference will never hurt the diff! because the spider gears are rotating slowly in the cage. I have seen where the pin in the spider gears has turned blue from spinning and heat, plus worn and loose. ALSO... if this was such a big deal the company's would not be giving you a wheel barrow tire as a spare on a new vehicle!
Click to expand...

The wheelbarrow spare is an interesting subject, too. Unbeknownst to me (prior to a few days ago), the Owner's Manual specifies that you cannot use that wheelbarrow spare on the front. If you get a flat on the front, you have to use the spare to replace a rear tire, followed by putting the rear tire on the front. That said, in my ignorance, I drove a few miles with the spare on the front without catastrophic failure.
Click to expand...

I probably was not in the market anyway, but you guys have sure convinced me to never buy a Subaru. Too many constraints and too much damage if you, say drive too far on a low tire. My wife would never understand it was low.

By the way, why not adjust the tire pressures to hard on three and soft on the big tire so you get the same rotation? I bet it would not be very many psi.

Or, buy one more big tire and put it on a different axle. There is a lot more differential rotation normally left to right than what you were getting, unless the Subarus are only for straight roads. Are we sure this is not a NASA design?
 
(quoted from post at 22:18:01 01/25/19)
(quoted from post at 20:05:51 01/24/19)
(quoted from post at 19:58:36 01/24/19) NOT SO !the only time the spider gears spin at a high rate of speed is when one wheel is stationary and the other one is spinning as when a person gets stuck n the mud. one size tire difference will never hurt the diff! because the spider gears are rotating slowly in the cage. I have seen where the pin in the spider gears has turned blue from spinning and heat, plus worn and loose. ALSO... if this was such a big deal the company's would not be giving you a wheel barrow tire as a spare on a new vehicle!
Click to expand...

The wheelbarrow spare is an interesting subject, too. Unbeknownst to me (prior to a few days ago), the Owner's Manual specifies that you cannot use that wheelbarrow spare on the front. If you get a flat on the front, you have to use the spare to replace a rear tire, followed by putting the rear tire on the front. That said, in my ignorance, I drove a few miles with the spare on the front without catastrophic failure.
Click to expand...

I probably was not in the market anyway, but you guys have sure convinced me to never buy a Subaru. Too many constraints and too much damage if you, say drive too far on a low tire. My wife would never understand it was low.

By the way, why not adjust the tire pressures to hard on three and soft on the big tire so you get the same rotation? I bet it would not be very many psi.

Or, buy one more big tire and put it on a different axle. There is a lot more differential rotation normally left to right than what you were getting, unless the Subarus are only for straight roads. Are we sure this is not a NASA design?
Click to expand...

Yea, I was unaware this car is so extremely fussy. I thought about the tire pressure approach, but decided it may not work.
 
.......

Or, buy one more big tire and put it on a different axle. There is a lot more differential rotation normally left to right than what you were getting, unless the Subarus are only for straight roads.....
Click to expand...

Not a good solution either. A larger tire on one side of the axle means the differential is slowly turning basically 100% of the time. For an open differential this is pretty meaningless. The problem occurs when you have a limited slip differential, particularly the viscous coupled design.

The percentage of time that the differential turns in normal situations with equal size tires is much less, only when going around corners. Most of the time your car is going in a fairly straight line so little to no differential motion is occurring.

All of this discussion is only relevant if there is a notable difference in tire circumference on a vehicle with limited slip type differentials.

I have a couple dead viscous couplers from my center differential on my car. From drag racing. On launch, the weight transfer causes the rear wheels to bite harder than the fronts which try to spin. The viscous limited slip tries to still direct some power to the rears though but in doing so suffers some abuse. After a few dozen passes, the viscous unit is pretty well of cooked.

Understand that this is a drivetrain that was designed around a 195HP engine. Last time I had the car on the dyno it was 430HP at the wheels so something around 500 at the crank. So not a weakness as designed. The really fast guys replace the viscous coupler with a solid one, but I use my car on the street still and a locked center diff is a PITA normal driving.
 

I recon you would have to live in the snow belt to understand why they have such a good group of Subaru lubbers ... Subaru drive-lines are not the issue that's what makes them what they are they are notorious for head gasket and oil leaks... They are fixable but it takes a dedicated man to resolve the issues...

When they first came out they were stingy on fuel I have know folks to get 40MPG on some of the 70's models... I brought my first one for $50 it was a trade in at the local chebby dealer. The head gaskets were blown the engine was a snap to get out almost as EZ as a bug... My BIL and I just reached in there and pulled it out by hand I had $50 in parts and 5 Hr. of my time I flipped it for $650 that was good money around 1980...

I had a bud that worked at a Subaru dealer they never went over well around here that's all he would drive he said they were as good a car as any... He was as good a auto trans mechanic as I have never know he told me about going to Subaru school he ended up telling them how to rebuild them... All in said and done he said he had never seen a Subaru trans give any issues so out in the wild he never got to repair one...
 
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