Do you really have to replace all four tires with AWD?

"Camshaft Chris: Skyline/McLaren/Porsche Fanboy" (camshaft-chris)
10/10/2019 at 16:14 • Filed to: AWD, Tires, New Tires, Ask Oppo

Pic for attention.

Helping a co-worker with new tires. I’ve never owned an AWD car before, so I’ve never heard this personally. But she was told at the tire shop that they had to do all four at the same time with any AWD car. Is this true? I mean, I guess it makes sense in my head, that if two wheels are constantly rotating at a different speed than the other two, that’s going to strain the drivetrain since they’re not all spinning equally. Anyone ever had experience replacing just two tires for an AWD car? Or is it better to just suck it up and do all four?

DISCUSSION (57)

OPPOsaurus WRX > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:20

anything i’ve heard agrees with that. I ha d a nail in a tire in the WRX and it was really close to the sidewall that they were concerned it wasn;t repairable and said I would have to do all 4 due to the AWD. Luckily it was repairable. beyond that I’ve always heard its true but I have no sources to support that claim right now.

CarsofFortLangley - Oppo Forever > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:20

You don’t have to replace tires until they fall off.

Don’t fall for BIG TIRE’s scams.

bob and john > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:21

yes, the tires have to be within a certian % for wear and tear so hat you dont over work t he center diff and fuck it.

facw > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:23

Yes, the tires need to be sized the same, so unless they are basically brand new, you should replace them all. If you just have an repairable flat or something like that, you can get a new tire shaved to match the others, but it’s probably not worth doing if the tires are old, or if you are replacing multiple tires.

NKato > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:26

If the manufacturer spec for the tires are the same, then it shouldn’t have a significant impact. The only thing you should worry about is tire wear. If they are all uneven, then yeah, that could pose an issue.

Personally, the four tire replacement on an AWD is more for peace of mind and min/maxing. Having a consistent set of tires is better than mismatched. It also can affect MPG.

I’m not gonna tell you to do one thing or another. Draw your own conclusions.

SBA Thanks You For All The Fish > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:26

I have several quattros and theoretically the Torsen diffs require that the circumference be within a certain fractional percentage in order not to cause excess wear in the driveline.

Having said that, if I still have at least 65% of original tread I never think twice about just changing one tire.

Wrong Wheel Drive (41%) > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:28

I am of the opinion that tires on most cars should only be replaced in sets of 4. This is assuming a proper rotation schedule and not getting a flat from a road hazard. But on AWD cars, its more about the differential setup and how exactly the system works. Subarus? Especially WRX/STi? Yup, gotta replace all four unless the tires are brand new almost. Its not worth risking a diff over trying to cheap out on that. Usually if its not road hazard related though, the other tires are doubtfully any good soon anyways.

On both my Miata and STi I have only ever replaced all four and they are usually all completely bald or corded by then anyways.

vicali > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:28

Yep, had a stem valve go low on the Forester and ended up running the tire for 200ft while flat. Cut up the inner cords and wrecked the tire. Four new ones $800 later..

Cheaper than a new center diff though.

vicali > CarsofFortLangley - Oppo Forever
10/10/2019 at 16:29

Ha! Is that your professional opinion?

Rusty Vandura - www.tinyurl.com/keepoppo > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:30

That’s a great question, thought I don’t think we have the definitive answer here. My initial reaction was, Great way to sell tires! but the argument about wearing out the differentials is a compelling one.

CarsofFortLangley - Oppo Forever > vicali
10/10/2019 at 16:30

*glances at truck with tires from 2007 on it*

Uh...kind of.

shop-teacher > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:31

Yes, because the circumference/diameter of all four tires needs to be the same. The alternative is to have the new tire( s) shaved down to match the wear of the old tires that are remaining on the car.

Ash78, voting early and often > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:32

I have trouble believing that the difference in rotational speed (or circumference) based on the tread depth alone is enough to cause havoc with the diffs. In terms of all the variables that cars face, it just seems tiny. Maybe the equivalent of a few psi low in one tire? And most TMPS won’t freak out over that.

Still, I do subscribe to the “replace both tires on the same axle” which is just a good policy for traction, as well.

And for most modern AWD cars that don’t keep all four corners engaged at once, I’m even more skeptical of any long-term damage.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > facw
10/10/2019 at 16:33

The tires are about three years old, so yeah, definitely not worth shaving, lol.

TheTurbochargedSquirrel > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:33

Always change all 4 tires on an AWD car unless wear is within a couple %. You can grenade differentials within a couple thousand miles if the tires are missmatched. Sometimes you can get away with it but it is not worth the risk in my opinion. If you want to replace 1 or 2 tires buy ones identical to what is already on the car and have them shaved to match the wear of the other tires.

TorqueToYield > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:34

Every AWD car manufacturer has a spec for how different the tire diameter can be. Check the manual or google it. Tire rack says for Subaru within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

Tire Rack (and I assume other tire sellers) will shave tires for you, but I have no idea how well that works.

If you’re replacing tires just because theyre old and you need new tires or something, not a puncture, yeah def do all four.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > bob and john
10/10/2019 at 16:34

That’s kinda what I was thinking. Makes sense I guess. It’s just really annoying that if one tire is crap, all four have to go.

bob and john > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:35

the worst is if the tires are in the middle of their lives.

if its near ther end, its ahhh, screw it. if its near the start, you can slap a new one on no problem.

In the middle you are le fuckered

Long_Voyager, Now With More Caravanny Goodness > Wrong Wheel Drive (41%)
10/10/2019 at 16:36

My fronts tend to wear long before the rears.......

Kinja'd!!!

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > NKato
10/10/2019 at 16:36

The tires she currently has on the car are three years old (came new on the car in 2016), so they’re somewhat worn down.

Thomas Donohue > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:38

Isn’t that what tire shaving is actually for? If the remaining three tires are at 50% of tread or better, you can measure them and then have the new tire shaved to be closer to the other 3.

You can do some more research, but Tire Rack offers this:

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18

Edit: Adding some manufacturer specs via TireRack

Here are recommendations from some of the manufactures that Tire Rack currently serves for matching the tires used on their four-wheel drive and all-wheel drive vehicles. Additional recommendations from other Original Equipment Vehicle Manufacturers is pending.

Audi As published in their vehicle owner’s manual, “rolling radius of all 4 tires must remain the same” or within 4/32-inch of each other in remaining tread depth.

Porsche Cayenne within 30% of the other tire on the same axle’s remaining treadwear.

Nissan GT-R when replacing less than four (4) tires, each tire continuing in service must have at least 6/32 inch (5 mm) of remaining tread depth.

Subaru Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > Ash78, voting early and often
10/10/2019 at 16:38

I agree that it seems like it shouldn’t cause THAT much difference in rotational speed. But it also makes sense that any difference could cause excess strain on the differential. And with this being a co-workers car and not my own, I don’t want to advise anything that could risk a very expensive repair down the road.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > shop-teacher
10/10/2019 at 16:40

The tires are three years old as it is, so I don’t think it would be worth it to shave down a brand new tire that much.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > TheTurbochargedSquirrel
10/10/2019 at 16:41

Tires are three years old, so I don’t think shaving a brand new one would be worth it.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > Thomas Donohue
10/10/2019 at 16:41

Tires are three years old, so I don’t think shaving a brand new one would be worth it.

HammerheadFistpunch > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:44

Depends on the system and depends on the tread depth. Technically speaking you should never replace just one tire on a drive axle because the differential will be constantly in motion. But thats a bit extreme. With on demand AWD system (like 95% of crossovers) I think its of minimal concern. With permanent 4wd systems (and most subarus) its probably a good bit of advice.

Bryan doesn't drive a 1M > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:44

I’m not sure whether it matters or not, but I know some places like Tirerack have the option to shave a new tire down to the tread depth of the rest of the used set. Seems a little wasteful, but could save a lot of money, especially if there is a lot of tread left on the remaining tires.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > bob and john
10/10/2019 at 16:44

The tires on this car (Rav4) are three years old at this point, so they’re in the “le fucked” stage.

Thomas Donohue > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:45

I’d balance that with the wear and mileage. You can still get 2-3 more years with most compounds. Also check the manufacturers recommendations, might only need to replace two on the same axle.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > HammerheadFistpunch
10/10/2019 at 16:46

Yeah, this was debating replacing two vs four. And it’s a 2016 Rav4, so I would assume it’s just an on-demand system instead of full-blown permanent AWD system.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > Bryan doesn't drive a 1M
10/10/2019 at 16:47

Current tires are three years old, so I don’t think shaving down a brand new tire would be worth it.

HammerheadFistpunch > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 16:50

If it were me and the tires were as old as you say. I’d just buck up and get 4 tires. something nicer.

BoxerFanatic, troublesome iconoclast. > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:02

Tire shaving may be a slight option for replacing a flat tire on an otherwise recent set, without wasting three good tires... but that is shaving very little off the replacement tire.

Cutting down most of the tread on a new tire to make it match 3 other old tires, would be very counterproductive.

A very mild AWD system that is rear-wheel-assist at low speeds, and disabled at high-speeds, like MINI’s all4 or something... probably could get away with two newer tires on the front, and recent tires on the rear but probably not significantly old ones or different tire size... where the front weight bias would flex the front tires more than the rears, and the viscous coupling is disengaged most of the time anyway.

Something more sophisticated, like some Jeeps, Audi, Subaru, Acura SH-AWD, and others that are constantly varying torque and reading minute differences in wheel speed should get four tires that are the same circumference.

The computer would read the difference in circumference as a difference in wheel speed, and try to constantly apply corrective torque split, and the system in general would generate more ambient heat.

Heat is what makes viscous couplings tighten and work, but if their duty cycle isn’t intended to be constantly engaged, that level of heat and engagement could induce more wear than intended, and cause reliability problems over time.

Also a reason to keep tire inflation in mind, a consistently low tire, or tire pressures that vary more than a couple of pounds from each other, and the recommended inflation pressure could cause similar driveline issues.

thejustache > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:05

I will say I have done this once, with no issues over the last year. But it was a particular situation. I bought a used ‘ 12 Tucson from my wife from a wholesaler. Car looked great, but the 2 front tires were shot while the rears were still in decent shape. I wanted to replace all 4, but was short on funds after buying the car and ended up just replacing the front 2. I have not noticed any issues yet over the last year+ of driving, but I’m pretty sure it is an on demand/fwd biased system so I could have just gotten lucky.

I’ve always replaced 4 tires at a time with my Subaru

someoneatacura > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:08

We replace 2 on the axle if the tread is more than 2/32nds different between t he tires. Everyone here is referencing worse case scenarios, which seldom come to be. I’ve seen customers come in with 1 new 10/32nds tires, and 3 3/32nds tires, and nothing was wrong on the car mechanically. If all the tires are around 5/32nds or less, might as well get new ones, as they really start giving up the ghost by that point.

AMGtech - now with more recalls! > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:10

General rule of thumb is that the tires need to be within about 3/32" of each other in tread depth of replacing less than four.

If they are not, differentials/transfer case will be constantly going at different speeds which can lead to premature wear. I have personally seen this lead to shuddering transfer cases on various makes, particularly when there is a clutch pack or viscous coupling used instead of a planetary gear set.

Most of the time, no harm is done. But I wouldn't take the chance.

ZHP Sparky, the 5th > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:11

Yeah learnt this the hard way when I bought my ’05 Legacy GT wagon. The day after I bought it went on a roadtrip from the Bay Area all the way up the Oregon Coast and back with my dad. 4 th of July, middle of nowhere got a flat on one tire. Turned out car was too low for factory jack (was on aftermarket suspension), so had to wait for a good Samaritan to come help out (AAA basically said they don’t know when they’d get to us). Got donut on finally and limped our way to our campsite ~50 miles away. Next morning was somehow worse when I took it in to a Les Schwab to just get the tire patched/replaced, and the rep told me they can’t do that, gotta do all 4.

Only tire they had in my size was a Federal S595 – people love it (cos it looks “aggressive”), biggest load of junk I ever spent $500 on. The guy felt bad for me so he gave me a $50 credit for my old tires that were in OK shape. Was a great road trip, but along with the tire scenario and unknowingly overtaking a cop on the freeway and getting pulled over…wasn’t the best introduction to the car.

But yeah - in cases where there is a diff between the wheels even if it’s not a full time AWD system probably better off replacing them all (I always dreamt of having a full size spare to keep in rotation as partial insurance, but sold the car before that ever happened) instead of putting unnecessary strain on the system.

Just Jeepin' > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:16

Another good reason, I suppose, to go with 5 tire setups and keep the spare in constant rotation. If you have to replace one, you can use the new one solely as a spare so you can keep using the other 4.

TheRealBicycleBuck > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:26

When my WRX was wrecked, the insurance only wanted to pay for one tire. I had to prove to them that replacing all four was required. So, I did my research. The first place I looked was in the owner’s manual. It is stated clearly that all four tires must be replaced at the same time.

Anecdotally, I found a post by a fellow in a forum who claimed he damaged a single tire while on a long trip. He replaced just the damaged tire. One hundred and fifty miles later, he had to replace a transmission.

Here’s a similar discussion: https://forums.nasioc.com/forums/showthread.php?t=2766137

shop-teacher > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 17:32

Agreed, I would just go ahead and replace them all.

TheRealBicycleBuck > Ash78, voting early and often
10/10/2019 at 17:35

The thing is, with a lot of cars, it’s not simple mechanical differentials, it’s clutch packs. With active torque management and active stability control, the wheel speeds are constantly monitored and the clutches and brakes are applied accordingly to get the desired torque distribution. When two wheels are constantly moving at different speeds, the active systems work hard to correct that discrepancy and generate a lot of heat in the process.

Wrong Wheel Drive (41%) > Long_Voyager, Now With More Caravanny Goodness
10/10/2019 at 17:36

Thats what rotations are for! Gotta do burnouts evenly.

Sammyno55 > Ash78, voting early and often
10/10/2019 at 17:48

I’m skeptical as well. A differential is designed to allow two wheels to spin at different speeds. The difference in speed has to be so small as to not matter in a typical RAV4. I'd say to put it on the rear axle (usually not driven in this vehicle unless a slip is detected).

Here’s math to back it up.* OEM tires are probably 225/65R17s. Average circumference with new (10/32") tread is 89.5". A totally worn tire with 2/32" tread would have a circumference of 87.9". Worst case difference is only 1.8%. Assuming a half worn tire, I think it’s less than 1%. I’m not sure how a differential world be damaged by that. I see a compact spare as a much larger difference.

*All numbers sourced from the internet. Math done with a calculator.

Bylan - Hoarder of LS400's > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 18:01

Definitely depends on the 4wd system and how advanced the diffs are. Obviously hav ing 2 wildly different sized tires could strain the diffs, because now 1 wheel on an axle or 1 axle as a whole is rotating at a different speed than the others. But doing some maths, a 225/60/R16 tire has a diameter of 26.6". Most new tires have about 10/32" of tread on them. So the difference between a brand new 225/60/R16 and a tire with say 5/32" tread left is .3125".

N ew tire: 26.6"

Used tire: 26.29"

Circumference New: 83.5"

Circumference Used: 82.55"

New tire rotations over a 1 mile distance: 758.8 Rotations

Used tire rotations over 1 mile distance: 767.53 Rotations

So 8.7 more rotations over a 1 mile trip on a used tire vs. a new tire. I think this would only really matter if your diff was welded or you have a VERY sensitive electronic full-time-AWD system.

I’ m very bored at work and this was interesting to do some maths on. Anyone have insights on if speed sensors or those fancy E-LSD’s would pick up on such a small difference?

Personally I’d just replace them all, especially if 2 of the tires only had 5/32" tread left. I also like all my tires to be the same model.

NKato > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 18:28

Do them all.

Kev > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 19:24

I agree that it depends on the tread depth and the system. Some fulltime AWD systems are incredibly finicky. Old Merc Mountaineers, for example, would wipe out the clutch in the transfer case, if you had mismatched (old/new) tires. I don't think a RAV4 fits that category. But it's not a bad idea. Especially if you live up north.

Manwich - now Keto-Friendly > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 19:49

Supposedly it has to do with increased wear on the differentials

Personally when I buy tires, I buy 4 just because I like to have consistent handling. And I do that even though I’ve never owned an AWD vehicle.

Dogsatemypants > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/10/2019 at 20:34

The idea is to reduce disparity fro m fro nt to rear to hopefully reduce stress to the center diff which compensates for the difference in rate of rotation. Regular rotations are also suggested.

GD _sti > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/11/2019 at 01:39

Oh look, the poors are at it again... Tires are always cheaper than transmissions, Subarus/ all time awd are not to be haggled with... The wife’s Si enna is awd, probably more lenient on that system since it’s on demand.

Long_Voyager, Now With More Caravanny Goodness > Wrong Wheel Drive (41%)
10/11/2019 at 06:50

Burnouts are rotations!

BigBlock440 > Thomas Donohue
10/11/2019 at 08:14

Wow, those specs are pretty loose. Not sure what Subaru’s thinking where 2/32 of tread makes 1/4 inch difference though.

Thomas Donohue > BigBlock440
10/11/2019 at 08:35

1/4 inch of circumference. 2/32 of an inch change in radius on an 18 inch tire will be right about a quarter of an inch.

BigBlock440 > Thomas Donohue
10/11/2019 at 08:46

Ok, that makes sense, I just assumed diameter because I don’t ever use circumference.

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > GD _sti
10/11/2019 at 10:01

Nothing poor about it. Just trying to help out a co-worker and make sure she’s not getting scammed by the tire shop. Rich people don’t get rich by spending money unnecessarily ...

Camshaft Chris: Skyline/McLaren/Porsche Fanboy > Bylan - Hoarder of LS400's
10/11/2019 at 10:04

We’re dealing with a 2016 Toyota Rav4 Hybrid, so I’m going to assume the AWD system is not using advanced diffs or anything like that, lol. Just a basic, probab ly FWD-biased system.

Dr.Jeckyl > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/11/2019 at 11:48

I’ve heard this is the case for Subarus but I've never personally owned an AWD.

Harpoon > Camshaft Chris: Skyline/McLaren/Porsche Fanboy
10/11/2019 at 12:20

Simple. The answer is no. However, you should replace them in pairs (fronts/rears).

Think of it this way. Your rears will wear out faster than the fronts. After 5000 miles (or 10000 or 20000 or whatever), on a square setup (fronts same diameter as rears), the rears will be smaller than the fronts. Are you doi ng any harm to your vehicle by continuing to drive it? No.