Just Made a Total Newb Mistake

"RightFootDown" (rightfootdown) 11/06/2014 at 16:55 • Filed to: None

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Just bought my WRX and decided that since I have two left over Dunlop Winter Sport M3 tires from my E46 M3 that I would simply purchase two more to have a complete set. Didn't factor AWD in to equation. Now I have an unusable set of four winter tires.

UPDATE: To clarify, the old set is worn to about 6-7/32nds while the almost new set it 11/32nds. My understanding is while 4/32nds doesn't sound like much it can put a large strain on the drivetrain.

Am I wrong or am I actually okay?

Siiiiiiiiigh

//Follow me on Twitter @JoshTaylorUSA

mcseanerson > RightFootDown 11/06/2014 at 16:56

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I'm confused... What's the problem?

Decay buys too many beaters > RightFootDown 11/06/2014 at 16:57

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Sorry, why? Why would AWD make a difference?

HammerheadFistpunch > RightFootDown 11/06/2014 at 16:59

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different brands/models? as far as the treadwear goes (assuming they are the same make/model/size of tire) you have 2 choices:

1. Not care and run them anyway

2. Have the newer tires shaved to match the older ones.

With the WRX it shouldn't matter a ton front to rear, but it will matter a little side to side, especially in the back.

lonestranger > RightFootDown 11/06/2014 at 17:00

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Are the two pairs different sizes and/or models? Is the leftover pair significantly worn?

If not, what's the problem?

notacarguy > RightFootDown 11/06/2014 at 17:04

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they look functional to me!

Jordaneer, The Mountaineer Man > RightFootDown 11/06/2014 at 17:04

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erm, doesn't AWD have a center differential? I thought different sized/treadwear tires only mattered on 4WD cars

Luc - The Acadian Oppo > RightFootDown 11/06/2014 at 17:07

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What's the problem?

RightFootDown > HammerheadFistpunch 11/06/2014 at 17:09

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Old set of tires are 6-7/32nd tread depth. The barely used tires are 11/32nds. So I'm okay to go then?

HammerheadFistpunch > RightFootDown 11/06/2014 at 17:12

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yeah, don't sweet it

RightFootDown > lonestranger 11/06/2014 at 17:12

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Old pair are 6/32nd. New set are 11/32nd.

Boxer_4 > RightFootDown 11/06/2014 at 17:20

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From what I've seen, the general rule of thumb is a 2/32" difference left to right, and a 3/32" difference front to rear (both in terms of tread depth) is acceptable. That typically translates into 1/4" difference in the entire diameter of the tire being acceptable. While the difference isn't out of spec by too much, it is still out of spec.

Also, since your WRX is new and still under factory warranty, it might would be worth it to run four matching tires. I'm not saying that it would cause a problem, but if a problem does arise, the tires aren't then a reason why your warranty could be denied for some reason.

If you do decide to run these tires as is, keep the tires with equal tread wear on the same axles.

Trunk Impaired 318 > RightFootDown 11/06/2014 at 17:24

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Is the WRX 50-50 AWD?

Boxer_4 > mcseanerson 11/06/2014 at 17:34

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Subaru's AWD system (and most everyone's, I'd assume) adjusts the default torque distribution based on wheel speed. Theoretically, since the tires would have different diameters and would be rotating at different speeds, the AWD system would need to compensate (it thinks the wheels are slipping due to the different wheels speeds), which could prematurely wear the system a bit.

The obvious case would be running tires significantly different diameters, but running the same tires with different tread depths can also be an issue. The general rule is 1/4" difference, but right around that mark is really splitting hairs.

Boxer_4 > Trunk Impaired 318 11/06/2014 at 17:36

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The 2015 WRX with the 6 speed manual still uses the Continuous AWD setup with 50/50 default split.

mcseanerson > Boxer_4 11/06/2014 at 17:42

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I seriously doubt that a difference in tread depth would make a significant difference.

lonestranger > RightFootDown 11/06/2014 at 17:49

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Yep, I'm with Hammerhead on this. Yes, your centre differential is going to be operating more often than it normally would be in order to overcome the difference in circumference between the two sets of tires. I don't think it's not enough difference to worry about., but I'm not an expert.

If you still had the M3, I'd suggest doing a burnout on the new pair to shave 'em down to match ;)

Soloburrito > RightFootDown 11/06/2014 at 17:54

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http://www.tirerack.com/tires/tiretech…

There are several suggestions that have been offered to drivers who are replacing pairs or individual tires on their four-wheel drive and all-wheel drive vehicles. Some vehicle manufacturers recommend that all tires maintain the same rolling radius and circumference, while others suggest that all tire circumferences remain within 1/4- to 1/2-inch of each other. Other vehicle manufacturers recommend that all four tires remain within 2/32-, 3/32- or 4/32-inch of each other, or within 30% of each other in relative remaining tread depth.

!!! UNKNOWN CONTENT TYPE !!!

As an example of different tire diameters resulting from tires worn to different tread depths, we'll compare two 225/45R17-sized tires, a new tire with its original tread depth of 10/32-inch and a second tire worn to 8/32-inch of remaining tread depth. The new 225/45R17-sized tire has a calculated diameter of 24.97", a circumference of 78.44" and will roll 835 times each mile. The same tire worn to 8/32-inch of remaining tread depth is calculated to be 1/8" shorter with a diameter of 24.84", have a circumference of 78.04" and will roll 839 times per mile. While the difference of 1/8" in overall diameter doesn't seem excessive, the resulting 4 revolutions per mile difference can place a continuous strain on the tires and vehicle's driveline. Obviously, the greater the difference in the tires' circumferences, the greater the resulting strain.

RightFootDown > lonestranger 11/06/2014 at 18:18

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The burnout definitely crossed my mind. In fact, I should have offered a free tire shaving services on the side.

RightFootDown > Soloburrito 11/06/2014 at 18:19

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This is the exact article that made me throw my hands in the air.

AMGtech - now with more recalls! > RightFootDown 11/06/2014 at 21:22

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If it were mine I would just run them.

However... I have seen transfer cases and center differentials ruined by doing exactly this. It's somewhat rare but it does happen. From what I've seen, the clutches in Subaru center diffs have a tendency to wear out under normal operation anyways, so you might be asking for trouble here. The main reasons I would do this on my own car are that the clutches/diffs aren't crazy expensive at my cost and they're fairly easy to swap out, couple hours for me.

Raphmoe > Jordaneer, The Mountaineer Man 11/06/2014 at 22:05

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There is a rear differential.

Jordaneer, The Mountaineer Man > Raphmoe 11/06/2014 at 22:09

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I know, but I thought even then, it was hard on 4WD cars to be driven on the street. thats why my car is usually in 4WD auto.

Raphmoe > Jordaneer, The Mountaineer Man 11/06/2014 at 22:11

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What car?

Jordaneer, The Mountaineer Man > Raphmoe 11/06/2014 at 22:12

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One with dat ass...

okay a 2000 mercury mountianeer

Raphmoe > RightFootDown 11/06/2014 at 22:14

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Well, the worst that could happen is you'll have to replace your differentials on a new car.

Rico > HammerheadFistpunch 11/06/2014 at 22:46

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Just curious do you mean the 2 worn tires should be placed on either the front only or the rears only? And that pairing them with the less worn tires instead would be more of a strain on the differential?

HammerheadFistpunch > Rico 11/06/2014 at 22:57

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my thinking is that the vc unit in the rear diff is more likely to be stressed by different sizes than the vc unit in the center diff.

Boxer_4 > mcseanerson 11/07/2014 at 00:46

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That's probably true, but Subaru had to draw the line somewhere for specifying what they consider the recommended allowable difference in wheel/tire diameters. The 1/4" guideline is most likely fairly conservative, but there are other factors at play, too (tire pressure, vehicle loading, tire manufacturing tolerances, etc). Although, they probably have a small effect alone, too. I'm guessing that Subaru chose to have a smaller range on the tire diameter in order to allow a bigger range on the other factors, which could be more difficult to control.

William Byrd > RightFootDown 11/07/2014 at 07:55

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N00b!