AWD Spare Tire Size

[asanelli]

So I have searched around on the internet, but I cannot get a conclusive answer what is the maximum percentage of difference a tire can be when used with AWD as a spare?

I have read all things from 1% to 2% to 3%.

Does anyone know what the percentage is?

I want to get 255/75/17 (32.05") but I have a 265/70/17 (31.6") spare. The percentage difference in these two tires is, 1.41%.

Will I have issue?

 

[derekjmiller913]

Wondering the same.

"Will I have issue?"

Been wondering this myself! I have a set of 255/75/17 Duratracs with a slightly larger true Outer Diameter of the 255/75/17 BF Goodrich Mud Terrain I bought used for my spare. The difference is minor but still present and I wonder also if this is bad? Good question!

 

[Bumbo]

Its a good question but it actually depends...

Those who own a Torsen center probably have more allowance than some others. By design it allows for some difference is ratio between wheels before it starts to lock up.

Its how / why those of us with selectable 4WD can still drive on pavement in 4, with the center diff unlocked.

I don't know the numbers... but it could be as much as a 3:1 ratio, I think. That said, I may be confusing that with a Torsen style front diff and not the TC.

While the tire diameter matters, I think we get more wiggle room than people let on, but I don't know for sure.

If I needed a spare that bad, I would use it... that said my OCD doesn't let me NOT have a matching spare on the same OEM rim.

 

[Folsom4Rnr]

So I have searched around on the internet, but I cannot get a conclusive answer what is the maximum percentage of difference a tire can be when used with AWD as a spare?

I have read all things from 1% to 2% to 3%.

Does anyone know what the percentage is?

I want to get 255/75/17 (32.05") but I have a 265/70/17 (31.6") spare. The percentage difference in these two tires is, 1.41%.

Will I have issue?

Subscribed...

If I needed a spare that bad, I would use it... that said my OCD doesn't let me NOT have a matching spare on the same OEM rim.

LOL... +1 on that!
I am right there with you on that.

 

[Keros]

Supposedly the Torsen center diff is identical to that used in the Quattro.

Hence:
quattro allowing different roll diameter [Archive] - StanceWorks
Quattro Tyre Treads and Rotation - Possible Damage to Diff? - Audifans.net
https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=18&currentpage=52

Seems to be a substantial amount of common practice being that the tires should be the same size/type and similar wear. Considering the cost of a new t-case or repair, I don't think I'd think it's worth it to mess around.

 

[03v8raleigh]

They have to be very similar tires to not risk damage on dry pavement. What I did was just go to a used tire shop and find one similar, with similar wear, since mine didn't come with a matching 265/70/17 spare. Cost me $50 total. To have a match. That being said, I'm not sure if that larger tire size will fit in the spare location. Mine barely fits as is.

 

[derekjmiller913]

Mine barely fits as is.

Same. My 255/75/17 barely fits too. (same approx OD as the 265-70-17). The example in my photo is a 255/75/17 BFGoodrich Mud Terrain like from a stock Wrangler Rubicon. *The tight spot is actually closest to where the tire would meet the hitch, not the box shown.*

 

[Gavino18]

Whether you come into issues down the road is really up to a lot of factors, which you can totally avoid by just buying a used spare that is the same tire and similar tread.

Big difference in tread depth and pattern is when you start running into issues, but if you use it just as a spare in hairy situations you're fine. Just don't run different tires and tread for your setup.

 

[asanelli]

Same. My 255/75/17 barely fits too. (same approx OD as the 265-70-17).

These size are different by about a 1/2".

 

[derekjmiller913]

These size are different by about a 1/2".

You're right. What *especially* blows my mind is how different two separate 255/75/17's can be with their differing TRUE OD measurements. It's crazy! I guess it all has to do with tread depth and such...? :redbrick:

 

[Inv4drZm]

I guess I'm wondering what the problem is with running 2 different sizes on the same axle.

As long as the center diff and front/rear diff is unlocked you should be fine correct? The differential (by its name) allows two different shaft speeds so you can turn on concrete. A larger tire will rotate slower so if you are driving straight your differential would act as if you are turning slowly one direction. Where does the problem occur?

 

[green_manalishi]

I guess I'm wondering what the problem is with running 2 different sizes on the same axle.

As long as the center diff and front/rear diff is unlocked you should be fine correct? The differential (by its name) allows two different shaft speeds so you can turn on concrete. A larger tire will rotate slower so if you are driving straight your differential would act as if you are turning slowly one direction. Where does the problem occur?

The smaller wheel has less resistance and therefore becomes the driven wheel. Less resistance means the power is routed to this wheel the same as when one wheel of an axle is on ice and the drive shaft will spin up at the same rate as this frictionless wheel.

The issue with this doesn't occur at the rear differential, since it is open. The problem happens at the center differential due to the limited slip. With one smaller wheel on one side of the center diff, both front and rear output shafts are constantly turning at a different speed which activates the limited slip mechanism. Limited slip mechanisms wear a bit when activated.

Turning corners or driving on a slightly smaller spare tire for a short time won't cause much wear. Doing this continuously for too long will cause more wear.

The relevant questions, then, are: "How small is too small" and "How long is too long."


I linked a really cool video here showing a disassembled 4th gen transfer case and Torsen differential which may be helpful in this discussion.