Do AWD Cars “Require” 4 of the same exact tires when replacing them?

[ChrisL]

WOChrisL’s front tires are pretty worn.   I wasn’t checking tire pressure & damn I caught it too late…

I went to the tire place to replace the front tires and the guy says AWD cars require 4 of the exact same tires as it can ruin the AWD system.  They didn’t have the OEM tires in stock so he was trying to sell me 4 of a very similar tire.  I got 2 of tires instead as I didn’t buy what he was telling me but I thought I’d do some research as well.  

I get that a different size tire or say 2 off road tires and two road tires could be problematic but two tires of the same size with slightly different tread pattern?

Anyone have any insights on this?

 

[jsharr ★]

I will ask my boys

[jsharr ★]

[bikeman564™]

Reason I think the answer is yes is this. All vehicles have a speed sensor on each wheel. This is how ABS & traction control work. If two of the tires are worn, their diameters will be slightly different, which could cause issues w/ the aforementioned, or the AWD engaging.

[jsharr ★]

Just spent a few minutes googling. If one tire is brand new and another tire has almost no tread left, the tires will rotate a different number of times per mile due to the difference in diameter. This can cause strain as the all wheel drive system is trying to spin tires at different speeds for a prolonged period of time. Consumer reports even mentions that you should consider Tread shaving to make all tires close to the same diameter. 
this basically means you buy a new tire and then have the tread depth shaved on it until it matches the other three tires.

[ChrisL]

5 minutes ago, jsharr said:

Just spent a few minutes googling. If one tire is brand new and another tire has almost no tread left, the tires will rotate a different number of times per mile due to the difference in diameter. This can cause strain as the all wheel drive system is trying to spin tires at different speeds for a prolonged period of time. Consumer reports even mentions that you should consider Tread shaving to make all tires close to the same diameter. 
this basically means you buy a new tire and then have the tread depth shaved on it until it matches the other three tires.

Thanks I was seeing the same.  The rear tires still had a lot of life left on them but we can afford two new tires, probably not a new transmission… I’ll likely replace the rears soon even though they are still good.

[donkpow]

"Tire Rack can provide a solution by matching the tread depth of the replacement tire to the tread depth of the partially worn tires that will remain on the vehicle by removing tread rubber from a new tire on a specialized machine that operates as a tire lathe (shaving a tire). While this may seem counterintuitive, the value of the mileage sacrificed by the one replacement tire is considerably less than the cost of rebuilding worn driveline components.

Tire Rack's tire shaving service is primarily used to prepare competition tires for race track use. This same service can also be used to remove tread rubber from new pairs or individual street tires used on four-wheel drive and all-wheel drive vehicles to allow them to match the remaining tread depth of the other partially worn tires that will remain on the vehicle. In addition to providing equivalent tread depth to eliminate driveline stress, shaved tires will also better match the traction and handling qualities of the remaining worn tires.

While the cost of our street tire shaving service will range from $25 to $35 for each tire, it is significantly less than the cost of unnecessarily replacing the remaining two or three good tires with lots of mileage still available from them."

 

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=259

[petitepedal ★]

The dealership couldn't save a tire that had a screw in the sidewall for one of my residents..the option was replace one and "shave" the wear to match the other 3...or 4 new tires...he got 4...he his license was renewed for 4, he has 4 new tires..now he said.."I hope I have 4 more years"

Oh and this was told to me Fiday morning...he had it done Thursday.

[Ralphie ★]

One of the many reasons I wish my wife would have bought an accord instead of crv. 

[donkpow]

Modern AWD cars are computer controlled. When the computer sees a difference in speed between tires under given conditions, it will apply the brake (through the ABS) to slow the faster tire down. Normally offsetting spin. In the case where tires are of a different diameter sufficient to influence what the computer senses, it will apply the brake(s) to correct the condition. IOW, the system might lock up one or more of your tires unexpectedly. That would be bad in a turn.

[Wilbur ★]

I always replace all 4/5.  

[Rattlecan ★]

All wheel drive cars have a third differential to differentiate between the axles much like tandem drive trucks do. To have different size tires front and rear means that the third differential is constantly making up the difference just like the right/left differential would if you were constantly rounding a curve. While it will accelerate wear somewhat, it would take a while to make a noticeable impact. 

[Ralphie ★]

I thought I had found that it is only “recommended” for optimum tire wear or some such. I think a thread aboot it is in here somewhere. Brb!

Found it, but nothing conclusive in it.

 

[Further]

I'm gonna say the AWD system in your car is what matters, I think there are different systems and some of them (probably the newer ones) tire diameter does matter. Older systems aren't so picky. My 03 Element I didn't replace all the tires at every tire change. 

[Dottleshead]

No. AWD just means the car is driven by two axels instead of one. So maybe it makes sense to replace the two on the same axle. I always did that anyway 

[Bikeguy]

If the car is a Subaru with AWD, then YES the tires all need to be the same size.  

If I ever have a flat tire, and I need to install the temporary spare tire.   I need to move a fuse to de-activate the AWD system, and it becomes a FWD car.   If I recall, if the flat tire is on the front of the car, I need to move a tire from the rear to the front and then but the temp tire on the rear. 

If you only purchsed 2 new tires, they always go on the rear of the car.  

 

[ChrisL]

24 minutes ago, Dottles said:

No. AWD just means the car is driven by two axels instead of one. So maybe it makes sense to replace the two on the same axle. I always did that anyway 

Yeah that’s a given.  The issue is what others have alluded to with the AWD system compensating for the different sized tires albeit maybe 1/8 - 1/16 of an inch.  I measured it with a key on the tread marking the top of the new tire. When doing the same with the old tire there was a difference but not much.  

I’ll likely replace the rear tires soon but not this weekend.   

[Dottleshead]

1 hour ago, Wilbur said:

I always replace all 4/5.  

I do also but if I didn’t, I’d buy at least 2 on the same axle.

[maddmaxx ★]

3 hours ago, ChrisL said:

WOChrisL’s front tires are pretty worn.   I wasn’t checking tire pressure & damn I caught it too late…

I went to the tire place to replace the front tires and the guy says AWD cars require 4 of the exact same tires as it can ruin the AWD system.  They didn’t have the OEM tires in stock so he was trying to sell me 4 of a very similar tire.  I got 2 of tires instead as I didn’t buy what he was telling me but I thought I’d do some research as well.  

I get that a different size tire or say 2 off road tires and two road tires could be problematic but two tires of the same size with slightly different tread pattern?

Anyone have any insights on this?

 

Any time you purchase just 2 tires you risk (but don't necessarily get) a change in balance and handling.  An extreme example would be 2 new tires in the front and 2 almost baldies in back.  When you have to hit the brakes hard you may find yourself backing into what you were stopping for.  

Next situation is that not all tires of the "same size" are the "same size".  Construction differences may result in minor differences in roll out (distance the tire goes in one revolution).  The same would happen if air pressures were noticably different between tires...........bad handling.

Difference in tread design would also affect the handling.

As to how much the differences in similar tires would affect the all wheel drive.............if all 4 have about the same rollout, probably not much.  If they have a different rollout then something mechanical has to slip somewhere to make up the differences.  That's about all I know about 4wd.

[Bikeguy]

5 hours ago, ChrisL said:

 The issue is what others have alluded to with the AWD system compensating for the different sized tires albeit maybe 1/8 - 1/16 of an inch.

THIS...

I was surprised to find that the tire circumference on a Subaru needs to match within 1/4 of an inch.  (or less).    I follow Subaru's recommendations.  I figure they designed the AWD system and know what will hurt it.

From the Tire Rack web site.

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.

That said...   I LOVE how my Subaru Forester and the winter rated all season Michelin CrossClimate2 tires can travel thru snow in the winter.    It's WAY better than most cars.  

I replace all 4 tires when needed.  They are THE only thing that connects you car to the road.  Therefore they are the most important for my car repair criteria. 

My criteria for importance of car repair is;

1) stopping

2) steering

3) everything else

This has worked for me for many years.