Proper Use of Mismatched Spare Tire

[BillyHW]

Hypothetically speaking,

let's say you have a spare tire that was never used because you never included it in your rotation, or maybe you even have a spare tire that was nominally the same tire size code, but is a different brand/make/model of tire,

and let's say that your regular 4 tires are pretty worn down,

and then let's say you get a flat tire.

What would be the proper procedure for putting on the spare?

In this case your spare tire might be quite a bit different size than your remaining 3 good tires.

Can someone confirm that this would be the correct procedure and would be safe without damaging anything:

1. If the flat is on a rear tire, move a good front tire to the back and mount the spare tire to the front.

2. Drive home only in 2WD (=RWD on the Jeep). Do not engage 4WD until you get equally sized tires on all four spots again.

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[nota4re]

The danger of a different size tire is that the rotational speed difference may "fool" a limited slip differential into locking up when it shouldn't. If you do not have a limited slip differential in the rear of the Jeep, then you have nothing to worry about. (Limited slip in the front is NOT even an option.) Typically the diameter difference between a worn tire of the same size compared to a relatively new tire is not going to be enough of a difference to cause a problem with a limited slip differential. For sake of argument, let's say you purchased and installed (4) 35" tires and your spare is a 33".

1. You could install the spare on the front without (mechanical) consequence but obviously you are going to have to watch your speeds and suspension behavior - and it will likely have a pretty good "pull"..... but you could nurse it to a tire repair shop.
2. You could put the spare on the back without consequence if your Jeep does not have a limited slip differential option. In fact, if you don't have limited slip and have a front tire failure (and depending on the distance you need to drive), it might behoove you to put one of the rear tires up front and let the diameter difference exist solely on the back axle.
3. The 4WD system is not going to care about diameter differences. You could use that as you please without (mechanical) consequence.

 

[BillyHW]

The danger of a different size tire is that the rotational speed difference may "fool" a limited slip differential into locking up when it shouldn't. If you do not have a limited slip differential in the rear of the Jeep, then you have nothing to worry about. (Limited slip in the front is NOT even an option.) Typically the diameter difference between a worn tire of the same size compared to a relatively new tire is not going to be enough of a difference to cause a problem with a limited slip differential. For sake of argument, let's say you purchased and installed (4) 35" tires and your spare is a 33".

1. You could install the spare on the front without (mechanical) consequence but obviously you are going to have to watch your speeds and suspension behavior - and it will likely have a pretty good "pull"..... but you could nurse it to a tire repair shop.
2. You could put the spare on the back without consequence if your Jeep does not have a limited slip differential option. In fact, if you don't have limited slip and have a front tire failure (and depending on the distance you need to drive), it might behoove you to put one of the rear tires up front and let the diameter difference exist solely on the back axle.
3. The 4WD system is not going to care about diameter differences. You could use that as you please without (mechanical) consequence.

Okay, so it's the limited slip that matters and not the drive axle? But what about the locked center transfer case in 4WD? Do the open diffs on each axle allow for any variations at the wheels to cancel each other out so there are no mechanical problems created?

I always heard that you should have 4 equal tires for AWD/4WD.

 

[nota4re]

Hi Randy - I think you are mostly all correct. The only exception would be the case you cited for yourself. *IF* you have a limited slip differential and have a 33 on one side and a 31 on the other, you are really asking for problems. (None of this discussion has anything to do with LOCKING differentials.)

 

[BillyHW]

But what about the locked center transfer case in Part Time 4WD?

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