Can one new tire hurt the differential

I bought a used 2017 Legacy Limited 2.5i, the dealer had it inspected before I bought it and I took it to another mechanic to give it a second inspection. The mechanic said that the rear left tire was newer than the rest and had more tread depth. The “Miles Left” he put on each tire in the inspection sheet are as follows:

 

LF: 20,000 RF: 22,500 LR: 55,000 RR: 22,500

 

He said I could damage the differential if I continue to use a tire with a different tread depth, and he wants to swap out the tires for new ones as soon as possible. Articles on the internet seem to support his view that different tread depths pose a problem for AWD vehicles in particular.

 

What do you guys think, do I need to swap them out immediately, or can I get away with another year or so on these ? Anyone ever damaged their differential with one new tire?

Yes, it will hurt the AWD and it should be swapped as soon as possible to prevent any further damaged to the AWD clutches. You can have a tire shaved to match the depth of the other 3 worn tires.

Thanks! I looked into shaving and called a few tire places and doesn’t seem to be something very many places do anymore around here, the Boston area, especially if you don’t buy the tire you intend to shave from the place. One guy even claimed it was illegal because you can make bald tires look new (?). My mechanic said my current tires are cheapo anyway, Goodyear eagle brand and have bad reviews, I might just want to bite the bullet and get 4 new ones.

Maybe just get one used tire for now to replace the new tire for now till your ready to buy a new sets of tires.

dgoodhue is correct. Mismatched tires can damage the AWD system, specifically the Multiplate Transfer Clutch. Subaru's published limit for tire mismatch is 1/4 inch in circumference ... which works out to about 1.3/32 difference in tread depth.

 

For completeness, the front and rear differentials probably won't be hurt by running mismatched tires, just the MTC, which functions as the center differential.

 

My advice: Buy a new set of four tires, and be done with it. You might ask the seller to share the cost, since they sold you a car with mismatched tires way beyond Subaru's published limit.

Edited July 22, 2020 by ammcinnis

dgoodhue is correct. Mismatched tires can damage the AWD system, specifically the Multiplate Transfer Clutch. Subaru's published limit for tire mismatch is 1/4 inch in circumference ... which works out to about 1.3/32 difference in tread depth.

 

For completeness, the front and rear differentials probably won't be hurt by running mismatched tires, just the MTC, which functions as the center differential.

 

My advice: Buy a new set of four tires, and be done with it. You might ask the seller to share the cost, since they sold you a car with mismatched tires way beyond Subaru's published limit.

 

Thanks so much for these numbers, ammcinnis. The mechanic didn't write down the specific tread depth, only those "Miles Left" on the inspection sheet. He verbally told me the tread depth and I don't remember what he said, kind of assumed he'd write it down, but it was definitely more than 1.3/32.

 

I only bought the car a month ago. Armed with Subaru's published limit, I called the dealer who sold it to me, a Cadillac dealer by the way, and complained that the mismatch was beyond the limit. His service department acknowledged that a mismatch causes issues especially for AWD and he offered to take it in and "fix it." Taking it in on Tuesday and we'll see what they do.

Glad to hear they're going to fix it; however, I'd find out what the "fix" is before taking it in to them. They may end up putting four used tires on there that have the same tread depth. Depending on the tires, that may be a good thing or a bad thing.

 

/

 

There's no such unit as "miles left".

 

The only helpful units are of tread depth in 32nd of an inch. We can infer from what the tech gave you that the tires are severely mismatched, but I find it odd that he "dumbed it down" to "miles left" for you, as if you couldn't compare something like 4/32" to 8/32" on your own.

 

Heck, he could have done the math for you re: remaining tread depth and given you a % of tread left, which isn't as helpful as having the actual tread depth numbers, but more helpful than "miles left".

There's no such unit as "miles left".

 

The only helpful units are of tread depth in 32nd of an inch. We can infer from what the tech gave you that the tires are severely mismatched, but I find it odd that he "dumbed it down" to "miles left" for you, as if you couldn't compare something like 4/32" to 8/32" on your own.

 

I agree, I thought that was weird too that he would go out of his way to convert to “miles left” and leave out the actual measurements on the sheet. He apparently does this with all inspections because “miles left” is a field on the sheet, he didn’t just write it in. Either he wants to show customers what the numbers really mean in terms of longevity and not bother them with the back end stuff, or he doesn’t want them to do what I’m doing now lol, going to the dealer with precise numbers and having them fix the issue, because he wants to sell 4 new tires. Maybe that’s too cynical, I’ve bought tires from him before and he seems generally fair. Still business is business.

Armed with Subaru's published limit ...

Here is one citation from Subaru's Tech Tips newsletter:

 

 

Here is one citation from Subaru's Tech Tips newsletter:

 

Nice! Good to get something from the horse’s mouth.

 

By the way, after some googling I discovered that a tread depth gauge costs five dollars on amazon and can arrive Friday. Since two parties with interests not entirely aligned with the customer are involved, I’ve decided some objectivity is in order.

Nice! Good to get something from the horse’s mouth.

 

By the way, after some googling I discovered that a tread depth gauge costs five dollars on amazon and can arrive Friday. Since two parties with interests not entirely aligned with the customer are involved, I’ve decided some objectivity is in order.

 

 

Good job, bud.

 

I still don't understand his intentions. Is he looking to get himself sued? How in the hell would he know how much life is remaining? Does he take into account the wear rating, 200 vs 800, manu vs manu? It's probably just an eyeball assessment, which isn't helpful.

 

/rant

Nice! Good to get something from the horse’s mouth.

 

By the way, after some googling I discovered that a tread depth gauge costs five dollars on amazon and can arrive Friday. Since two parties with interests not entirely aligned with the customer are involved, I’ve decided some objectivity is in order.

 

you could also just use anything that is narrow enough to fit between the thread and mark using a pencil/ sharp tip marker etc and measure using a regular desk ruler etc (I've used a credit card and a marker) so you don't have to wait.

 

Like others have said, it amazes me how they arrived at how many miles are left Even tire manufacturers only put approximate numbers and they know exactly what compound is being used...

 

I've always been weary of used tires when I get a new to me vehicle after experiencing a blow out and a 180 on the interstate on a car I picked up the day before... You never know how well or poorly they were maintained through their prior life. And you can usually pick up a decent set of 4 for ~500 bucks. It's cheap peace of mind imo. That's just me; don't mean to scare you.

 

See if they will let you check the manufacturing week/year on the tire before they put it on, if they do agree to give you a full set. I've had one dealer try to sneak 5 year old tires that they had sitting in their warehouse. They were new (ish) but the rubber compound breaks down faster than you think with fluctuations in temp and humidity

 

Good luck at the dealer.

 

I've always been weary of used tires when I get a new to me vehicle after experiencing a blow out and a 180 on the interstate on a car I picked up the day before... You never know how well or poorly they were maintained through their prior life. And you can usually pick up a decent set of 4 for ~500 bucks. It's cheap peace of mind imo. That's just me; don't mean to scare you.

 

See if they will let you check the manufacturing week/year on the tire before they put it on, if they do agree to give you a full set. I've had one dealer try to sneak 5 year old tires that they had sitting in their warehouse. They were new (ish) but the rubber compound breaks down faster than you think with fluctuations in temp and humidity

 

Good luck at the dealer.

 

 

Great advice. Yeah I’ll be getting specifics on what they’re going to put on the car if they replace anything. I’d rather get nice tires even if I have to share the cost, than get cheapo or worse, used ones put on.

The difference between left and right rear tread depth looks to be about 3/32 “ judging by pen marks on an old credit card measured by a ruler.

 

So if that’s accurate it’s well over 1.3”/32.

Edited July 23, 2020 by wick3rman

get yourself a tool to measure tread depth and might as well get a tire pressure gauge as well. it should be part of everyone's toolkit.

 

 

let us know what the selling dealer's remedy was.

 

 

I haven't had to ever replace just one tire with my awd drives so it's always been replace in pairs or replace all four and be done with it.

get yourself a tool to measure tread depth and might as well get a tire pressure gauge as well. it should be part of everyone's toolkit.

 

 

let us know what the selling dealer's remedy was.

 

 

I haven't had to ever replace just one tire with my awd drives so it's always been replace in pairs or replace all four and be done with it.

 

Glad you posted chowser, I was just about to follow up with the results. The tool came from amazon, I put it on those little square things between the treads and looks like it confirms what the credit card guessed, a difference of 3/32 ". Here are that mechanic's delightful "Miles left" compared to what the my gauge says in 32nds of an inch:

 

LF: 20,000 RF: 22,500 LR: 55,000 RR: 22,500

 

LF: 3/32 " RF: 3/32 " LR: 6/32 " RR: 3/32 "

 

Now on to the dealer... I'm leaving the car there Tuesday evening, he's giving me a loaner car in the meantime while he "fixes" it. I'll make sure I figure out what that means before I let him.

Wow. 3/32. I’d be replacing those 3 tires soon. I usually start shopping replacements at 3/32nd. Especially if winter is coming.

... I put it on those little square things between the treads ...

Were you perhaps measuring at the wear bars instead of measuring the full tread depth?

I haven't had to ever replace just one tire with my awd drives so it's always been replace in pairs or replace all four and be done with it.

In the Subaru AWD system, replacing tires in pairs is generally just as bad as replacing them one at a time. It's usually all or none. All tires need to be kept within 1/4 inch in circumference of each other.

 

Of course, replacing tires in pairs is common practice in 2WD vehicles with open differentials.

Edited July 26, 2020 by ammcinnis

Were you perhaps measuring at the wear bars instead of measuring the full tread depth?

 

Is that what those square thingies are? I think I must have misintepreted something I read to say you should measure depth from there. I'll have to measure again to get the real depth. Thanks for pointing that out, amcinnis.

 

I guess the depth of all of them will be a bit more, but the difference should be the same as above.

Where you measured is a visual indication of the minimum legal read depth. So, you’re likely to REmeasure at 4-5/32.

 

I usually start shopping for new tires at 3/32nd, as well, but would do so sooner if I didn’t have a dedicated winter set of wheels and tires.

Where you measured is a visual indication of the minimum legal read depth. So, you’re likely to REmeasure at 4-5/32.

 

I usually start shopping for new tires at 3/32nd, as well, but would do so sooner if I didn’t have a dedicated winter set of wheels and tires.

 

Exactly, the measurements turned out to be 4.5-5 / 32 on all three except LR which was 8/32.

 

Just got back from the dealer (a Cadillac dealer). Asked him what the plan was after he checks it out. He said he had no plan and would call a Subaru dealer and ask them What they normally do in situations like this. I guess a situation like this would not normally emerge in the first place for a Subaru dealer because they would not sell a car with tires that mismatched.

 

I reiterated to him the whole deal with needing to swap out all 4 tires every time even one tire needed replacing, because mismatches damage the “Multi-plate Transfer Clutch.” (Thanks am-c). Yes, I used that word and it sounded good in the moment though I have no idea what actually is or means. Google images of it look interesting. Also quoted the whole Subaru 1/4” rolling circumference = 1.3/32 max tread depth difference. His eyes glazed over.

 

I asked him if he would call me before doing anything to the car because I would rather have good new tires put on than used, and he assured me if he replaced any tires they would be new ones. That is reassuring.

 

The nice part about buying a used car from a Cadillac dealer is that they give you a CT 5 as a loaner while they do service. So I’m not super rushed about getting this issue resolved.

 

The nice part about buying a used car from a Cadillac dealer is that they give you a CT 5 as a loaner while they do service. So I’m not super rushed about getting this issue resolved.

 

LOL. Those things are nice while they are new

LOL. Those things are nice while they are new

 

Lol yes, looking into Cads as a car it seems they do not last in the long run. This one has a lot of annoying bells and whistles too: the seat vibrates under me when so much as a bird flies by, and the mirrors turn down for no reason when you back up. The side and rearview mirrors are useless anyway because they are so tiny. Did someone think that was a nice form factor? But hey, they fold up by themselves electronically... woohoo.

 

This dealership is taking forever to resolve the situation. I finally got a call yesterday saying they would not replace the tires and they suggest I get the tire shaved. I asked them if they had inspected it and measured the tread depth before selling it and they said yes and admitted their measurements were inaccurate: all equal at 6"/32. I said okay you made a mistake and now I have to pay for it before a component is damaged? Let's make this right and come to an agreement. Asked them what they would have done had they caught the mismatch, sold it as is anyway? Shaved one tire and put it on, which is what they are asking me to do now?

 

I would be happy with them giving me a rebate for whatever that would have cost (shaving new tire down, getting used tire-- they didn't have one) so that I could put it toward new tires. Not an outcome I'm particularly happy with but willing to settle. The service guy put on airs of being "caught in the middle" between me and the salesman who refused to do anything, since the car was "priced to sell" -- not true, it was pretty much the market value, maybe a little more. He then said he had called a Subaru dealer who told him that tire mismatches take a long time to cause any harm and that it's not unheard of to replace just one tire out of the 4 if the customer insists on doing so. That weak argument kind of pissed me off... sure, if the customer insists on doing something that could damage the car, there is nothing legally preventing you from doing it. No sh*t. Anyway he said he'd talk to the salesman again Monday.

Anyway he said he'd talk to the salesman again Monday.

 

Good luck! The sales folks aren't the easiest to deal with especially after you signed on the dotted line. And the manager may not care much either, coz they know they won't be making any money off of you by you taking the car there for service...