I am wondering this for rotating out the spare tire to make the tread depth the same as the others. Maybe I should order 5 tires instead of 4 and then rotate, rather than having a 15 year old spare under the back which is a different size than the others.

Alternatively, if you get a flat in the back, the solution would be to swap a regular size front wheel into the back and run the different size spare in the front.

I try to envision the diff turning. This is a good video for visualising it. YouTube



It seems like the gears would just spin slightly differently without causing damage. On a traction control 01/02, it might cause some problems, I don't know.

Sorry this isn't extremely technical but yes you definitely want to have a proper full sized spare. I thought about this a lot and it is one reason I moved my Savage rear bumber back 1.5" so that a full sized 33" would fit below. The main reason was that while in death valley a few years ago I blew a tire during my spindle failure and I was left with 3 tires that had about 50-60% tread. I ended up having to sell the 3 tires used and bought new tires because having one tire that was so much larger would be bad for the diff and transfer case (when 4WD is engaged).

Now that I am running 5 tires I get the following benefits:
* If I blow a tire (for me this is a super rare event) at least I have 4 good tires still
* Same sized spare = being nice to your diff and transfer case
* 5 tires rotated equal longer life per tire set

Supposedly it heats up the diff. I've run the same size but worn and unworn tires(micro difference in diameter) for a couple thousand miles on a long trip, and nothing bad happened.

For temporary use, its fine. Assuming it is the same tire 'size' and it is just tread depth differences...

It is never a good idea to run wildly different tire size, but it will at least get you off the trail - and maybe home if you don't drive to fast. In fact, I'd probably worry more about a 15 year old tire under the truck blowing when used than the diff... (unless you have an LSD - different story there!)

I put a used 265/75-16 BFG KO under my truck and my 4 main tires are 265/70-17 KO2's. I'd feel comfortable using it for a few hundred miles if I needed to get home (or to a tire shop) from the boonies. Before that, the spare was a stock-sized 265/70-16 very old tire, which I wouldn't want to drive on more than long enough to get to the closest tire shop...

-Charlie

Yes different size tires can put a strain on the diff and transfer case. I learned that with a 4wheel drive chevy, went and got some used studded tires for the back end. truck howled like crazy, went back to the same tire shop, different guy, he looked and said who sold you these, you cant run different sizes in a 4wd. So he swapped them out. I did not know that then, it was my first 4wd.

So my 4runner
I had to get a new spare so got a new KO2. However all the rest are half worn so I don't want to use the new spare unless I have to. But when I get my next set of Ko2's I will start a 5 tire rotation. Temporary I have no problem using it if needed.


x wifes awd volvo, needed 2 tires, had to buy 4 so they would match in size. I think that is pretty common with anyone with a AWD

In 2WD micro-differences probably not an issue. But in 4WD having a single 31" tire and 3x 33" tires .... well I wouldn't risk it.

It wouldn't mke any difference at all, unless you lock the diff.

Heating up the diff?? What?

The differentials whole purpose is to allow the wheels on that axle to spin at different speeds (around corners). Having a different sized tired is basically just the equivalent of driving around a curved road. There would be no damage to the differential, as that is what it's designed to do.

The spider gears in the middle of the trans are normallynot moving much at all in normal use. They move a little as you turn a corner. And probably move slightly even in a straight line due to slight differences in tire diameters - even if similar wear and size.

But - and I'm no automotive engineer, mind you - they don't have the bearing capacity to be moving a LOT for a LONG period of time. They just ride on bushings, not ball or roller bearings. They certainly have a nice constant bath of gear oil, but still , they're smaller bushings. Compare that to the bearings on the pinion gear. And the diff carrier. Those rotate constantly, and are very large and substantial roller bearings.

Personally, I don't think having mismatched tires is much of an issue, as long as it's not done for very long. While the large bearings elsewhere in the diff may be good for hundreds of thousands of miles of use, I don't think the spider gears are.

On the other hand (lol, talking out of both sides of my mouth here) look at the way the ADD front diff works. It disconnects one side of the diff. But the other CV shaft is still locked in, and doesn't disengage. But disconnecting one side allows the ring and pinion (and driveshaft) to stop turning, but the spider gears themselves are constantly turning. There's practically no load on them, but still, they sit there and happily spin for hundreds of thousands of miles with no ill effect.

The above is just for 4Runners, and other cars with open diffs (the locker is an open diff when not locked). Many AWD cars have viscous couplings or clutch type diffs in the center which can be harmed by relatively modest amounts of tire size differences. Also, cars with limited slip or lunch-box style lockers - those are also not intended to have extended periods of slip without overheating and/or wearing out.

This reminds me of a problem I had last year. I have a 2001, and the spare had never been off the truck. I had a flat tire, so I decided to swap the spare on. I checked first, and the spare was flat, too. So I put the spare in the back of my Scion and drove to the nearest air pump. It was out of commission, so I drove several miles to the next one. I unscrewed the cap, which was a little tight. Finally got it to move, and the whole valve came out - rotted from exposure and lack of use. So now I have a truck with two flats. Decided to drive to the nearest auto parts store and picked up an $18 air pump that you plug into your cigarette lighter. Was able to fill the tire on the truck, and the next morning had my wife take the truck and the spare to a local tire shop for repairs.

So if you haven't had yours off in 15 years, check it - lol!

A helical/Torsen type LSD might be ok-ish with mismatch tires. A viscous or clutch-type LSD would need matched tires to avoid damage. Luckily, those are pretty rare on 4Runners.

(My AWD cars, on the other hand... LSD's galore! Lots of careful tire rotations with those)

-Charlie

Yeah, I see that a lot around here. Someone gets a flat on the highway and goes to put in the spare but the spare is completely flat and/or rotten. They then leave the car still on a jack on the side of the highway. People don't think about the spares for years until they need it. At least the OP is thinking ahead! Not many do that.

To the OP, a simple wearing of a few 1/32's of tread depth is not going to blow your differential. It's not enough of a difference to matter. It only starts to matter if it's several inches of difference, say a sock sized 31" matched with a 33". If it were the case, all of our spares would be useless!

THIS ^^

If you have a locking rear diff or LSD and a smaller sized spare and blow a rear tire, swap a good front tire to the rear and put the smaller spare up front