(Purchase) Two tires or four?

[an_asker]

OK, I'm getting "tire"d analyzing this, so here goes!
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We have a Toyota Corolla which, when I took it to Discount Tire recently for an air check up, I was informed is running low on front tire tread. In fact, the tech wrote up tread depths of 1/32" and 2/32" in the front two and he advised me to replace them asap.

Now, at the same time, he assessed the rear two at 5/32" and 6/32" (if I remember correctly).

Which of the following options is the best (given the above disclaimers, or in spite of them)?

- just replace the two front tires with new tires and wait until the rear ones are as bad before replacing them;

- replace the two front tires with some used tires and wait until the rear ones get bad before replacing all four with new tires at the same time, or

- replace all four tires with new ones right now and be done with it?
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Disclaimers/caveats/key info items out of the way:

- I hate wasting stuff (and don't want to throw away tires that are still not past their end of life stage)

- this is mostly DD/DW's car (so my miserliness shouldn't govern lol)

- the car currently has four Michelins (and has about 40k miles) and I desire to replace with Michelins (and at Discount Tire) only

- Discount Tire usually has a $100 off deal but only when you buy four tires

[Globalviewer58]

What tread depth on the spare tire? If you really want to pinch pennies use the current spare and a new tire on one axle. Buy a used tire to replace the spare.

If it were me I would replace all four tires as the car is driven by family members that are not easy to replace.

[an_asker]

What tread depth on the spare tire? If you really want to pinch pennies use the current spare and a new tire on one axle. Buy a used tire to replace the spare.

If it were me I would replace all four tires as the car is driven by family members that are not easy to replace.

I think I got my answer!

PS: spare is a donut. So not good as a longer term stopgap measure.

[armeliusc]

Replace all four now, and moving forward make sure the tires are rotated periodically so that you don't get uneven wear like now.

[Globalviewer58]

A learning opportunity: when tires have 3/32 to 4/32 tread depth difference on the set of four, they were likely not rotated on schedule which would give more miles to the set. Discount Tire usually rotates at no charge and often takes appointments so you don’t have to wait long.

[tetractys]

All 4. There isn’t enough life left on the back tires anyway to even think about it. With the new tires, take advantage of the free rotation so they wear evenly.

[Sic Vis Pacem]

If it goes between you and the ground, it's worth spending money on. Shoes, chairs, beds and... tires.

I'd replace all four. And rotate them more often to avoid that uneven wear.

[Onlineid3089]

I agree on replacing all and rotating tires regularly moving forward. I typically did mine every other oil change which put it around every 10,000 miles.

[Watty]

In your situation I would go on and replace all four tires to get the $100 discount, and this time I would rotate my tires better so that they would wear more evenly.

- I hate wasting stuff (and don't want to throw away tires that are still not past their end of life stage)

One way of looking at tires is how many tires you will buy while you own the car.

For example if you will likely replace the car when it has 30k more miles on it then you will likely need to buy four more tires no matter if you buy the extra two now or a year from now. Likewise if you sell the car when it has 70k more miles on then you will likely buy two more sets of tires while you own the car.

Assuming that you are not planning on selling the car in a few months there are very few scenarios where not buying the extra two tires now will allow you to avoid buying the same number of tires over the long run.

[an_asker]

OK it's unanimous. And yes, a slap on the wrist for not being pro-active with the tire rotation! Will be more mindful going forward. I need to do similar checking on other car now.

Definitely not selling the car. We run our cars to the ground. This is our newest car.

[tibbitts]

OK, I'm getting "tire"d analyzing this, so here goes!

It's really not worth spending time analyzing this since the differences in cost just aren't significant in the long run. Some factors that might help you decide to just get 4 new tires is if you can even match the tires you have any longer (with an identical model), and the age of the less-worn tires.

[anoop]

Replace all 4 and keep them rotated here on out.

[andypanda]

If you do buy 2 new tires, be certain they put them on the rear.

"Should two new tires go on the front or back?
When tires are replaced in pairs, the new tires should always be installed on the rear axle, and the partially worn tires should be moved to the front.

Where to install two new tires on your vehicle - Goodyear Tires"

[chuckwalla]

If you want to max out all the treads left, I would rotate the tires front to back now (X pattern), drive another 5-10K miles and then replace all 4 at that time.

[talzara]

If you want to max out all the treads left, I would rotate the tires front to back now (X pattern), drive another 5-10K miles and then replace all 4 at that time.

The OP has already exceeded the maximum tread life. Florida requires 2/32" of tread on all four tires.

[andypanda]

You would keep driving those two bald tires? We have too many thunderstorms for it. Thin tread also doesn't provide much protection from road hazards.

"the tech wrote up tread depths of 1/32" and 2/32""

The CrossClimate2 tires on my wife's Avalon came with 10/32"

The Michelin Defender LTX M/S on my 4Runner came with 12/32"

[ncbill]

If you do buy 2 new tires, be certain they put them on the rear.

"Should two new tires go on the front or back?
When tires are replaced in pairs, the new tires should always be installed on the rear axle, and the partially worn tires should be moved to the front.

Where to install two new tires on your vehicle - Goodyear Tires"

That's what I do with the minivan since I always forget to rotate...rear to the front, new tires on the rear.

Lather, rinse, repeat.

[LotsaGray]

If you do buy 2 new tires, be certain they put them on the rear.

"Should two new tires go on the front or back?
When tires are replaced in pairs, the new tires should always be installed on the rear axle, and the partially worn tires should be moved to the front.

Where to install two new tires on your vehicle - Goodyear Tires"

That's what I do with the minivan since I always forget to rotate...rear to the front, new tires on the rear.

Lather, rinse, repeat.

For a Corolla and most cars, if you are going to have new and old tires on vehicle, new ones should go on front. Particularly true with front wheel drive.

[LotsaGray]

If you do buy 2 new tires, be certain they put them on the rear.

"Should two new tires go on the front or back?
When tires are replaced in pairs, the new tires should always be installed on the rear axle, and the partially worn tires should be moved to the front.

Where to install two new tires on your vehicle - Goodyear Tires"

That's what I do with the minivan since I always forget to rotate...rear to the front, new tires on the rear.

Lather, rinse, repeat.

For a Corolla and most cars, if you are going to have new and old tires on vehicle, new ones should go on front. Particularly true with front wheel drive.

[andypanda]

No. Just no.

For safety, always put the 2 new tires on the rear.

"This is the same for vehicles with rear-, front- or all-wheel drive. Ideally, you'd replace all four tires. But if only two need to be replaced, the new tires should go in the back, says Michelin."

New Pair of Tires: Do They Go on the Front or Back? | Allstate

[andypanda]

"That's because when the rear tires on a front-wheel-drive lose grip, it's almost impossible to keep control of the vehicle. I have experienced this at Continental's test track in Uvalde, Texas. When you only need two new tires, put them on the rear axle.Dec 20, 2020

Why Two New Tires Always Go On The Rear Axle

Kenwood Tire Company"

[andypanda]

https://youtu.be/HdSf0KJie_E

3-minute video

"If you're changing only two tires, always install the two new tires on the rear axle in order to drive safely and avoid hydroplaning."

"Install Two New Tires on the Rear Axle

Michelin USA"

[suemarkp]

Not sure it really matters. Most tire companies are paranoid about oversteer (the rear end losing traction and you spin out) so they all recommend things to avoid that. Most people don't have much experience with oversteer and can't get out of it easily. But if the front loses traction first (understeer), you lose steering ability. So you can decide if you want to hit the wall with your front end or back end. I'd rather not hit the wall at all and have 4 mostly equal NEW tires.

[MathWizard]

I just did sets of 4 on both our cars.

Part of the issue was age of tires.
My wife's tires still had decent tread, but they were 9 years old.

[lazydavid]

For a Corolla and most cars, if you are going to have new and old tires on vehicle, new ones should go on front. Particularly true with front wheel drive.

No, no, absolutely not. If there's a significant disparity, tires with more depth go on the rear. Doesn't matter which wheels are driven. It's a safety thing, not a wear thing.

Now if the rears are 8/32 and the fronts are 7/32, by all means rotate them to even out the wear. But if the rears are at 5/32 and you're buying 2 new tires, the new ones go on the rear.

[Northern Flicker]

Replace the front tires and don't rotate the tires until the front have less tread depth than the rears. If the rears need replacing before that, move the fronts to the rear and put the 2nd pair of new tires on the front. Rinse. Lather. Repeat.

Most recommend rotating tires say every or every other oil change, putting the less worn pair on the front. This will provide more even wear. But since you don't have electronic AWD, you could just continue not rotating the tires, and replace the front pair and rear pair on separate schedules. That's probably what I would do if it were my car.

Don't buy used tires.

[Northern Flicker]

No, no, absolutely not. If there's a significant disparity, tires with more depth go on the rear. Doesn't matter which wheels are driven. It's a safety thing, not a wear thing.

I'm not convinced of that but this site agrees with you:

https://tiregrades.com/tire-maintenance ... t-or-back/

Doing that while replacing tires in pairs will mean always having lightly worn tires on the rear and increasing cost and consumption of tires, as the less new ones in the front will always be wearing out.

[Northern Flicker]

You could consider Riken tires. These use the moulds for Michelin tires that were dropped from the Michelin product line, but a polymer/rubber compound that will not last as long as a Michelin tire. (Riken is a Michelin brand). Example:

https://www.tirerack.com/tires/TireSear ... ue&tab=TDG

With a 300 treadwear rating, they may last about as long as your 5/32" depth Michelins, or about as long as a pair of used Michelins.

[an_asker]

OK looks like this issue equally divides Bogleheads lol!

I've already decided to get four new tires, so this is a moot point but ... I remember that Discount Tire and my trusted auto mechanic have different answers on the "where should (only) two new tires go?" I forget who says what but one place says they should go on the front and the other says they should go on the rear.

Needless to say, Discount Tire wins as they are the ones who actually do the job lol.

But it does look like there are enough Bogleheads to vote for front and for rear!

[retireIn2020]

OK looks like this issue equally divides Bogleheads lol!

I've already decided to get four new tires, so this is a moot point but ... I remember that Discount Tire and my trusted auto mechanic have different answers on the "where should (only) two new tires go?" I forget who says what but one place says they should go on the front and the other says they should go on the rear.

Needless to say, Discount Tire wins as they are the ones who actually do the job lol.

But it does look like there are enough Bogleheads to vote for front and for rear!

LOL, I just went through this, went into Firestone for 2 new tires. I'm in the waiting room and the salesman approached me and says, "If you buy a third tire, you get the fourth free". These where $250 ea. 10 ply HD pickup tires and I was only 50% tread on the 2 I planned to keep. thought about for at least 30-40 seconds and told him to replace all four (I wanted my free tire) LOL.

Oh, and when replacing only 2, the reason you put the new tires on front is because they are the most important should a blowout occur. Steering is way more important than getting traction.

[jlawrence01]

Personally, I replaced ALL four tires at the same time. The local Toyota dealership offers a "Buy 3, Get One Free" deal in March and October that make the prices more attractive than the local tire shops.

You might also look at replacing the donut spare as it is more than likely as old as the car. You do NOT want it to fail if you need it.

[JoeJohnson]

OP: Good choice on 4 tires. Good choice on getting rotations in the future (they're free at Discount Tire!)

In addition, visually look at all your tires a few times per year. 1/32" would be incredibly obvious. Look for cracking and dry rot. You might also consider buying $5 tire depth gauge.

[andypanda]

"Steering is way more important than getting traction."

How do you steer when the bald tires on the rear axle lose traction in a turn (or hydroplane) and the rear end of the car passes you as you're spinning out? It's simple physics. Really.

[rockstar]

Double check the thread depth. I’ve been told incorrect thread depths in the past. You can get a thread measure on Amazon for less the $5. If they’re really as low as they say, I’d replace them all.

[Northern Flicker]

"Steering is way more important than getting traction."

How do you steer when the bald tires on the rear axle lose traction in a turn (or hydroplane) and the rear end of the car passes you as you're spinning out? It's simple physics. Really.

If the front wheels lose traction, steering won't even change the trajectory of motion of the car. That's also simple physics.

It isn't safe to have bald tires anywhere on the car, period. That was not an option on the table. All four tires should be capable of good traction. Any that are not should be replaced. Having Michelin tires with 5/32" tread on the rear is not what I would call a safety hazard.

I hesitated to add this, but traction in the front also enables the technique of steering in the direction of a skid when a skid starts to steer out of it. Front wheel drive increases the effectiveness of this as well. But not all drivers will execute that well, nor should it be a justification for driving on bald or near-bald tires.

[Northern Flicker]

https://youtu.be/HdSf0KJie_E

3-minute video

"If you're changing only two tires, always install the two new tires on the rear axle in order to drive safely and avoid hydroplaning."

"Install Two New Tires on the Rear Axle

Michelin USA"

I think the video is nonsensical. If putting the new tires on the front leaves the car exposed to severe hydroplaning at 45 mph, then the existing tires being mounted on the rear in that case are unsafe. They should be recommending replacing them, not moving them to the front.

And what about someone who rotates their tires for even tread wear? They will be using tires well before replacement on the rear at roughly the equivalent tread wear to the front, and at the tread wear level they are saying is so susceptible to hydroplaning. Is Michelin really claiming their tires become unsafe on rear axles at around the halfway point of tread wear? Yikes. But that is the implication of the video. Having all 4 tires at around the same tread depth without new tires on the front does not make the rear tires safer. This all assumes a FWD car. One always would put the less worn tires on the rear of a RWD car.

This looks to me like a ploy to sell more tires, scaring people into buying 4 tires when they need 2. And with most 2WD cars being FWD, putting the older tires on the front will wear them out faster, leading to another tire purchase and the cycle continues.

My $.02.

[crinkles2]

I see no issue in replacing only the two tyres that need it.

FWIW while physics don’t care what region you’re in, every single tyre shop I’ve been to here in Australia laughed when I asked to put new tyres on the rear, their policy is a new pair goes on the front, full stop.

[the_wiki]

5/32 is already quite worn. If you only do 2, you’ll be back in a few months anyway. I’d do all 4.

[billaster]

I think the video is nonsensical.

I don't see what you are disputing. Anyone can see in the video with their own two eyes that you would rather be driving the car with the newer tires on the rear. Do you disagree? And the situation they are showing is to deliberately over drive to determine the point of hydroplaning failure. Any tire, even brand new ones will hydroplane if you push it hard enough. But the lesson is that given the same conditions you obviously have a greater margin of safety with the newer tires on the rear. This isn't complicated. Newer tires on the rear, greater margin of safety.

One of the reasons for this is the distribution of weight between front and rear. For most cars, and particularly front wheel drive cars, the weight is 60% on the front tires and 40% on the rear. More weight means better traction so the front inherently has more traction and the rear will have a tendency to hydroplane first at any given conditions.

This becomes even more extreme the moment you tap your brakes because that causes a forward transfer of weight, lightening the rear and making it even more susceptible to hydroplane skidding. Braking systems are designed to compensate for this somewhat by having a proportioning valve that applies more pressure to the front brakes than the rear brakes. But this just emphasizes the fact that the rear is generally going to lose traction first and that is why you want your best tires on the rear.

[billaster]

I see no issue in replacing only the two tyres that need it.

FWIW while physics don’t care what region you’re in, every single tyre shop I’ve been to here in Australia laughed when I asked to put new tyres on the rear, their policy is a new pair goes on the front, full stop.

Makes perfect sense because they drive on the opposite side of the road and hydroplane water circles counter-clockwise.

[talzara]

Is Michelin really claiming their tires become unsafe on rear axles at around the halfway point of tread wear? Yikes. But that is the implication of the video. Having all 4 tires at around the same tread depth without new tires on the front does not make the rear tires safer. This all assumes a FWD car. One always would put the less worn tires on the rear of a RWD car.

This looks to me like a ploy to sell more tires, scaring people into buying 4 tires when they need 2. And with most 2WD cars being FWD, putting the older tires on the front will wear them out faster, leading to another tire purchase and the cycle continues.

No, Michelin is saying that its tires are less safe at 50% tread. Tires are safest on wet pavement when they are new and have the deepest tread. Even if it still has 90% of its original tread, it is still less safe than when it was new. When you replace tires depends on your own personal assessment of risk. Some people replace tires at 4/32", and other people replace them at 2/32".

This does not assume a FWD car, although it is more important for FWD cars. The front wheels are also used for steering on a RWD car. The Michelin video is saying that you want to lose traction on the front tires first so you become aware of the danger. When you can feel it through the steering wheel, you'll slow down. What makes it safer is the feedback to the driver, not just the physical capabilities of the car or the tire.

[Northern Flicker]

I think the video is nonsensical.

I don't see what you are disputing. Anyone can see in the video with their own two eyes that you would rather be driving the car with the newer tires on the rear. Do you disagree?

The video shows one car with major hydroplaning at only 45mph, claims the tires are the problem due to their treadwear, and then suggests it is appropriate and safe to use the tires on the front axle. That is nonsensical. If the tires are the problem on the rear axle leading to hydroplaning at 45mph, then they are not safe on the front axle either. They could just as easily have shown a car with these tires on the front continuing to go straight despite the steering wheel turned left while driving on wet leaves on the road.

[tibbitts]

5/32 is already quite worn. If you only do 2, you’ll be back in a few months anyway. I’d do all 4.

Well, a brand new Michelin Premier has only 8.5/32", so 5/23" isn't super-worn.

[billaster]

The video shows one car with major hydroplaning at only 45mph, claims the tires are the problem due to their treadwear, and then suggests it is appropriate and safe to use the tires on the front axle. That is nonsensical. If the tires are the problem on the rear axle leading to hydroplaning at 45mph, then they are not safe on the front axle either.

Of course it's safe. They stated that the older tires are at half tread. Seriously, do you throw away your tires when they are half tread claiming they are unsafe? That would be nonsensical.

They are test driving beyond what would be a normal safe speed in a hydroplane condition to test the safety margins. What they are demonstrating is that you have more safety margin when the newer tires are on the rear.

Unless you always dispose of your tires at half tread, the demonstrated safest policy is to put the better tires on the rear.

[Northern Flicker]

The video shows one car with major hydroplaning at only 45mph, claims the tires are the problem due to their treadwear, and then suggests it is appropriate and safe to use the tires on the front axle. That is nonsensical. If the tires are the problem on the rear axle leading to hydroplaning at 45mph, then they are not safe on the front axle either.

Of course it's safe. They stated that the older tires are at half tread. Seriously, do you throw away your tires when they are half tread claiming they are unsafe? That would be nonsensical.

They are test driving beyond what would be a normal safe speed in a hydroplane condition to test the safety margins. What they are demonstrating is that you have more safety margin when the newer tires are on the rear.

The video claimed at about the 1:39 mark that the tires were safe on the front because the driver can compensate. Then toward the end they show both cars driving simultaneously-- i.e. not the same tires, so not a scientific, apples-to-apples comparison. We also have no way of knowing that the driver was not deliberately causing a fishtail skid. We also don't know that these are FWD cars. I think the video may be a staged advertising stunt designed to sell 4 tires instead of 2. I would need to see a more scientific comparison.

If you rotate tires and don't replace at half tread, you will be driving on rear tires in the same state.

[suemarkp]

I don't see what you are disputing. Anyone can see in the video with their own two eyes that you would rather be driving the car with the newer tires on the rear. Do you disagree? And the situation they are showing is to deliberately over drive to determine the point of hydroplaning failure. Any tire, even brand new ones will hydroplane if you push it hard enough. But the lesson is that given the same conditions you obviously have a greater margin of safety with the newer tires on the rear. This isn't complicated. Newer tires on the rear, greater margin of safety.

One of the reasons for this is the distribution of weight between front and rear. For most cars, and particularly front wheel drive cars, the weight is 60% on the front tires and 40% on the rear. More weight means better traction so the front inherently has more traction and the rear will have a tendency to hydroplane first at any given conditions.

This becomes even more extreme the moment you tap your brakes because that causes a forward transfer of weight, lightening the rear and making it even more susceptible to hydroplane skidding. Braking systems are designed to compensate for this somewhat by having a proportioning valve that applies more pressure to the front brakes than the rear brakes. But this just emphasizes the fact that the rear is generally going to lose traction first and that is why you want your best tires on the rear.

I think this is the real reason. Most drivers, especially US drivers, are not well trained. When most experience oversteer (fishtailing) they panic and hit the brakes. This makes the fishtail worse because of weight transfer to the front. Locked up brakes or tire grip that is all going to braking dont steer well either. Staying on the gas, or at least off the brakes, is counterintuitive to most people. Some may steer quick enough to control the skid, but most people dont practice this and you need to in order to do it without thinking.

You can see this every winter here with cars sliding downhill with the front wheels at full turn but the car wont turn. Get off the brakes and it will turn.

[billaster]

I think the video may be a staged advertising stunt designed to sell 4 tires instead of 2.

What? How? They are telling you specifically how to buy just two new tires and be safe.

If you rotate tires and don't replace at half tread, you will be driving on rear tires in the same state.

That's fine. But if you have a road damaged tire and choose to replace just two tires instead of all four, where do you put them? This demonstration clearly shows you have more safety margin if you put the new tires on the rear and less safety margin if you put the new tires on the front. It's just that simple.

[Northern Flicker]

I think the video may be a staged advertising stunt designed to sell 4 tires instead of 2.

What? How? They are telling you specifically how to buy just two new tires and be safe.

Well, I'm a bit skeptical that is what they are doing. I tend to believe the video was staged for the result. Tire manufacturers don't need to shoot a video with professional drivers on a track to make a recommendation for where to mount the tires. The video plants a seed of doubt about using the existing tires on the front by saying that when the half worn tires hydroplane on the front, the driver can respond and make adjustments. But if someone still buys just 2 tires, the half worn tires on the front of a FWD car will wear down quickly, needing replacement before long, leading to an additional tire purchase. (With a RWD car, wear considerations would lead to putting the tires on the rear anyway).

The telltale sign for me is that they make a point of showing the tires being mounted on the white car when they switch from the blue, as if doing a careful comparison but then show both cars together at the end. They are not being compared with the same tires at that time. And in that final scene where they want to show the cars going the same speed, it seems obvious to me that the blue car made a deliberate hard left turn, rather than experiencing a fishtail skid on a curve-- they weren't even on a curve at that time.

Not mentioned in the video, but it does make sense that the weight of the engine compensates for more tread wear on the front in terms of traction. That would support the claim in the video. The claim may be correct, but I don't find the video to be convincing.

Also, a logical inference from their claim would be that one never should rotate tires on a FWD vehicle, but replace the fronts as needed, putting the new pair on the rear. This would avoid or reduce the time of having half-worn tires on the rear. If the video is correct that half-worn rear tires reduce safety, then avoiding tire rotation on a FWD vehicle would be an important safety enhancement. They are not making that recommendation, however. Why not?

[andypanda]

"I think the video is nonsensical."

You are certainly welcome to cling to your own opinion. I'll go with what the tire manufacturer's recommend.

And I do get rid of my tires when they are half worn and three or so years old. The tire shop buys them and resells them. I use my 4Runner off road, including towing a 16' boat off road to fish in lakes with no ramps, and need traction in Virginia's red clay when it's wet. And in the winter with our black ice mornings and short snow events that feature refrozen slush. Our two lane roads out here in the country are often tree shaded with patchy ice.

Driving in dry snow is much easier what we usually get. Of course in central Virginia any dry snow is usually on top of a layer of ice.

My father was born in the mountains near Charlottesville and worked as a state trooper and then as a safety engineer for national trucking companies. When I left the DC area for VA Tech in 1968 he gave me a $20 bill for car emergencies and a set of chains. I no longer carry chains, but I do have a snatch strap and set of traction boards for driving on the beach.

[lazydavid]

We also don't know that these are FWD cars. I think the video may be a staged advertising stunt designed to sell 4 tires instead of 2. I would need to see a more scientific comparison.

I'm sorry, what? That is an absolutely ridiculous statement. In which alternate universe are a fourth-gen Taurus and fifth-gen Malibu not FWD cars?

Also, a logical inference from their claim would be that one never should rotate tires on a FWD vehicle, but replace the fronts as needed, putting the new pair on the rear. This would avoid or reduce the time of having half-worn tires on the rear. If the video is correct that half-worn rear tires reduce safety, then avoiding tire rotation on a FWD vehicle would be an important safety enhancement. They are not making that recommendation, however. Why not?

Going about it in this way, would in fact be a (sub-optimal) method of rotating. The significantly less-worn rears get moved to the front when the fronts get replaced. You don't buy more tires because total wear over time doesn't change, but you do buy them more frequently, because you're never buying 4 at a time. I would not suggest this strategy, but it would technically work. The risk is not specifically "having half-worn tires in the rear". It is having rear tires with significantly less traction capability than the fronts. Notice the word significantly in that sentence. You don't want rear tires that are dramatically more worn than the fronts, because you will not know about the consequences of that disparity until it's too late.

But that does NOT, logically conclude that tires should never be rotated. Let's keep that word "significant" in mind and do a little math, intentionally avoiding letting perfection be the enemy of very good. If a tire comes from the factory at 10/32nds, and must be disposed of at 2/32nds, there are 8/32nds worth of tread available for use. If the warranty is for 50k miles, then the expectation is that you should get about 6,250 miles per /32 on average. So if you rotate your tires every 5k as recommended, the tread depth will always vary by less than 1/32nd. Even if you stretch it to 10k, it will still be just over 1/32nd, and will even back out following the rotation.