Replace all 4 tires: Fact or Fiction

[tater_salad]

On other forums I always see people scream about replacing all 4 tires on an AWD car, and keeping them rotated all the time, or the Dif will die in 2 months.

I'm just not seeing it, I could understand if all 4 wheels were being driven all the time, but as far as I know Subarus are 2wd until the main wheels slip, then it powers other wheels with traction.

Also wouldn't AWD systems be designed to take care of the 1-2 inches of differential between the circumference of the tires. I could understand this not being worked out 25 years ago, or with something like a donut spare (2wd fuse). But not with same sized tires

[Suzam]

There are an amazing amount of discussions on the board about this. A search would provide a plethora of knowledge.

 

Suffice to say Subarus always provide some power to all wheels, usually 90% front & 10% rear. If 1 or 2 tires are of a different circumference the internal clutches (auto) or viscous coupling (manual) will always be working to compensate for the differences. The AWD system sees this a slippage. The greater the difference the quicker the wear.

 

So yes, replace all tires at once unless the difference in circumference is less than 1/4", as recommended by Subaru.

Edited October 25, 2010 by Suzam

[edrach]

On other forums I always see people scream about replacing all 4 tires on an AWD car, and keeping them rotated all the time, or the Dif will die in 2 months.

I'm just not seeing it, I could understand if all 4 wheels were being driven all the time, but as far as I know Subarus are 2wd until the main wheels slip, then it powers other wheels with traction.

Also wouldn't AWD systems be designed to take care of the 1-2 inches of differential between the circumference of the tires. I could understand this not being worked out 25 years ago, or with something like a donut spare (2wd fuse). But not with same sized tires

You asked the question.....the answer is fact. I don't know the cost of failure to adhere to that with an automatic transmission since I don't drive one, but the cost of replacing the center diff in a 5MT transmission is around $600 just for the parts involved. Add to that the cost of the labor (your own--zero--or a shop, up to $500 more or more); a set of tires are cheap by comparison.

 

By way, I have a set of three tires with good tread I can sell you real cheap.

[idosubaru]

this isn't a black and white discussion like some may tell, but it's certainly is a fact that replacing all 4 tires is wise.

 

subaru's are not 2wd, you can clear that up with some reading on how they work, lots of info out there on that.

 

Subaru has the specifications in the owners manual - measure the circumference and make sure they are within a certain limitation. most folks don't care to measure tires...i don't, so it's simpler to rotate tires properly and replace as a set.

 

will your trans experience issues with tires out of limits and no attention to rotation, yes. will it be 10,000 miles or 100,000 miles, no one can predict that. you're asking too much to quantitatively analyze every vehicle, every driver, every person's round trip to work, car useage, type of tires, rotation, air pressure...you see my drift.

 

so we get a one size fits all standard. yes it's conservative...how conservative is not a very profitable question since we're talking a big and expensive job if you have to pay someone to repair your trans.

 

if you're that bent against it, put your best tires up front and they'll eventually wear down to meet your rears.

[edrach]

grossgary makes a good case and argument. I can only speak for our experience with our '99 forester with the 5MT. We bought it used with 110K miles on it and the carfax we ran supported that mileage pretty well. It had four new tires on it when we got it so I couldn't tell what the earlier set looked like. At 125K we started to get the symptoms of torque bind and at 135K I bit the bullet and had the center diff replaced. I did search for about 3 months to find a used, good center diff but had no success so we went with a new one.

 

No telling why the center diff failed. It could have been that the previous owner had the car towed somewhere and the tow company didn't have the smarts to get all four wheels off the ground. Or the previous owner wasn't real conscientious with tire wear/rotation. A moot point since the failure happened.

[WJM]

Here's a good one:

 

2008 Legacy 2.5i sedan, 5MT.

Customer needed 2 tires as the fronts were TOAST from not rotating. Stock size is 205/50/17.

 

The tire shop installed two new 215/45/17's on the front.

 

600 miles later the center diff EXPLODED out of the bottom of the transfer case while driving down the interstate.

 

Needless to say, someone was buying an entire transmission.

 

http://www.miata.net/garage/tirecalc.html

^^^^^^^^^^^^^^^^^^^^^^^^^^

 

Is accurate.

 

25.1 inch diameter is stock for that vehicle, what was installed was 24.6 inch diameter. 1/2 inch of difference was all it took and BOOM.

[GeneralDisorder]

On other forums I always see people scream about replacing all 4 tires on an AWD car, and keeping them rotated all the time, or the Dif will die in 2 months.

I'm just not seeing it, I could understand if all 4 wheels were being driven all the time, but as far as I know Subarus are 2wd until the main wheels slip, then it powers other wheels with traction.

 

That is not entirely correct. The Automatic's are 80/20 till the front's start to slip then the rear is engaged by the TCU.

 

Manual's are entirely different (except the 6 speed's) - they are 50/50 but also have a center diff and viscous coupler that can heat up durring excessive wheel spin at one end or the other and transfer more torque to the end that had traction.

 

Also wouldn't AWD systems be designed to take care of the 1-2 inches of differential between the circumference of the tires. I could understand this not being worked out 25 years ago, or with something like a donut spare (2wd fuse). But not with same sized tires

 

Over time, differences in tire circumference will cause damage to the center diff/VC on the manual transmissions and will cause damage to the clutch pack on the automatics. It's a matter of them not being desinged to be in constant use - only used when turning and when wheels slip. The larger the differences in tire sizes, the faster these components spin while driving in a straight line, and thus the faster they overheat and wear out.

 

It's that simple. So YES you should always run similar tires (brand and size)with similar wear and inflation. You don't have to be super anal about it - within 25% tread wear and within 5 psi inflation is accurate enough to insure a good long life for your transmission.

 

GD

[edrach]

Here's a good one:

 

2008 Legacy 2.5i sedan, 5MT.

Customer needed 2 tires as the fronts were TOAST from not rotating. Stock size is 205/50/17.

 

The tire shop installed two new 215/45/17's on the front.

 

600 miles later the center diff EXPLODED out of the bottom of the transfer case while driving down the interstate.

 

Needless to say, someone was buying an entire transmission.

 

http://www.miata.net/garage/tirecalc.html

^^^^^^^^^^^^^^^^^^^^^^^^^^

 

Is accurate.

 

25.1 inch diameter is stock for that vehicle, what was installed was 24.6 inch diameter. 1/2 inch of difference was all it took and BOOM.

I should think the non rotation of the tires caused 50% of the damage to the center diff. Mounting the wrong size after that just compounded the problem.

[johnceggleston]

you can test it yourself, let the air out of one of your front tires, down to 20 lbs. or so and drive around town. see if you feel a difference.

[WRX2FFU]

I've a have a treadwear gauge. They are relativly cheap.

 

2/32 difference is a good rule, if you stay right there you'll most likley never have issues as long as your air pressures are good.

[WJM]

I should think the non rotation of the tires caused 50% of the damage to the center diff. Mounting the wrong size after that just compounded the problem.

 

It takes a lot longer on stock size tires, 2 worn and 2 not as worn in order to destroy things.