off sized spare? (1 Viewer)

[DetroitStyle]

Trying to keep costs down (aren't we all)... My 80 currently has a set of good tread, but dry rotted 265/70/16s on it.

I'm going to upgrade to 285/75/16s (no lift)... how bad of an idea is it to keep one of my 265s as a spare?

 

[DBWood]

Terrible for anything other than moving it around the yard or, onto a flatbed to tow it.
I don't know how to explain it technically, But, basically you will be asking the two axles that are attached to each other at the differential to turn at two different speeds.
Bad for gears, bad for rubber, bad for anything connected to them...

 

[IanB]

Bad idea due to the AWD, if you had a part time kit then it could be done.

 

[DetroitStyle]

Kinda what I figured with the fulltime AWD... Off to Craigslist to find a cheapy spare, I suppose.

Thanks!

 

[jonheld]

Terrible for anything other than moving it around the yard or, onto a flatbed to tow it.
I don't know how to explain it technically, But, basically you will be asking the two axles that are attached to each other at the differential to turn at two different speeds.
Bad for gears, bad for rubber, bad for anything connected to them...

Do you only drive in a straight line? What happens during a turn when all 4 wheels are turning at different speeds?

Differentials are there to differentiate. They're pretty good at that.

 

[Fitzer]

I think Jon is right. However I personally wouldnt want to run it that way at highway speeds for long distances. Most car's manuals say to use the same sized tires all around and at least the same size on an axle....probably to cover their butts, but maybe there is heat buildup on the differential splines if they are working hard... But also consider all the "Donut" spares out there for the flimsy sedans and I'm sure there has been a few where that donut stays on there until its bald working a sedan's flimsy diff. I'd probably be concerned with the dry rot moreso...especially if you keep it stowed under there for a few more years and then need it. You could probably find someone getting a new set of 285's and pick up a used left over one with low tread, but it would only be 4-5 years old and not dry rotted and it would be the right size (less the tread difference)

 

[DBWood]

Do you only drive in a straight line? What happens during a turn when all 4 wheels are turning at different speeds?

Differentials are there to differentiate. They're pretty good at that.

So, are you advising that is ok to drive with mix matched sizes of tires?

 

[DBWood]

Fitzer, the cheapo donut spares are always the same diameter as the factory tire size the vehicle came with. Just narrower, and definitely flimsy-er...

 

[starski]

Some tire shops sell used tires...I got a spare 285/75/16 for $30. Not the best tire but still some meat left on it..

 

[asutherland]

Do you only drive in a straight line? What happens during a turn when all 4 wheels are turning at different speeds?

Differentials are there to differentiate. They're pretty good at that.

It'd be like driving a highway with no straight stretches, ever. Not the end of the world. Like Jon says, that's what a differential is for. I would use it to get off the trail and to the next place you could buy a new tire, not leave it on there for weeks though.

Now, don't go put an odd sized tire on and lock it

 

[jonheld]

So, are you advising that is ok to drive with mix matched sizes of tires?

To be honest, with 3 open differentials I wouldn't give it another thought. Differentials are not made from eggshells. They are a series of large gears running in an oil bath inside a giant heat sink.
The original post was for an odd size spare that was smaller than the proposed tire size to be purchased. While the 1994 FZJ80 in question has a viscous center differential, the rotational difference between the left and right axle will be twice as much as the rotational difference between the front and rear driveshafts.

So unless you're running 33" tires all around and decide to put on 28" spare, I wouldn't really be too concerned. However it should be realized that the truck can possibly handle poorly.

And, no, I wouldn't run it for extended periods of time. But to say that you should only move it around in the driveway is a bit alarmist.

 

[ROCKWOOD21]

Noooo! Don't do it. I came on here and asked pretty much the same thing a while back. Everybody on here told me dont do it except for one of my buddies. I listened to him and destroyed my front and rear diffs along with the coupler. Costed me a lot of $$$. I was running 3 36's and one 35 for a few days. I understand our trucks our tuff but go find a matching spare. I promise it's not worth it. That was the biggest cruiser lesson I learned.. RUN ALL TIRES THE SAME SIZE or your Cruiser will teach you what I had to learn.

 

[retrofive]

Some tire shops sell used tires...I got a spare 285/75/16 for $30. Not the best tire but still some meat left on it..

I agree with Jon, however this is exactly what I did. $20 out the door. done. almost matches even.

 

[Pin_Head]

Here is an interesting story of what can happen driving long distances with a putatively siezed viscous coupler.

http://gearinstalls.com/tommyfzj80.htm

 

[asutherland]

Here is an interesting story of what can happen driving long distances with a putatively siezed viscous coupler.

http://gearinstalls.com/tommyfzj80.htm

I don't understand what that has to do with the OP's question?

 

[Splangy]

I don't understand what that has to do with the OP's question?

Viscous couplers don't like mismatched tires.

 

[asutherland]

Ok, fair enough... but according to the story on that page, it never had anything to do with mismatched tires?

The viscous coupler in the transfer case was believed to be mal-functioning and tried to lock up....

Sorry if I'm missing something here.

 

[Pin_Head]

They didn't mention tire size difference or whether they were the same, but something caused the VC to sieze and also caused a lot of stress on the front pinion. The VC was obviously siezed because the owner could drive it with no front pinion and without the center diff locked. That had to be due to a rotational difference between the front and rear pinions and there aren't that many possibilities: tire size difference or a constant turning. Front pinions don't just disintegrate for no reason and the suspect is a siezed VC. I thought it was interesting and relevant, but you are welcome to think otherwise.

 

[asutherland]

They didn't mention tire size difference or whether they were the same, but something caused the VC to sieze and also caused a lot of stress on the front pinion. The VC was obviously siezed because the owner could drive it with no front pinion and without the center diff locked. That had to be due to a rotational difference between the front and rear pinions and there aren't that many possibilities: tire size difference or a constant turning. Front pinions don't just disintegrate for no reason and the suspect is a siezed VC. I thought it was interesting and relevant, but you are welcome to think otherwise.

Don't worry about it dude, I was just curious how it was related.

I think the consensus is, the world's not going to end (neither will your VC) if you need to move your truck 50 feet on a smaller tire... but do yourself a favour and buy a full sized spare.

 

[Tools R Us]

They didn't mention tire size difference or whether they were the same, but something caused the VC to sieze and also caused a lot of stress on the front pinion. The VC was obviously siezed because the owner could drive it with no front pinion and without the center diff locked. That had to be due to a rotational difference between the front and rear pinions and there aren't that many possibilities: tire size difference or a constant turning. Front pinions don't just disintegrate for no reason and the suspect is a siezed VC. I thought it was interesting and relevant, but you are welcome to think otherwise.

There are a few things that are not mentioned, likely Zuk was/is unaware of. The rig was bought at a "deal", with issues, lack of maintenance, etc. Was hit hard enough to have a bent front arm, yes it had a locked VC, etc. The heat was likely from the "fluid" in the front diff, it had not been changed in a very longtime, was very thick, basically grease, this likely caused the gear failure.

Agree that it best to run the same size tires, at least close. But I have been running half worn on the front, with new on the rear, without issue. The VC can be overheated, fail from excessive differential speed for too long. Also spider gears don't have bearings, so don't tolerate, can be damaged by excessive speed for too long.

That said, what is the damage threshold? My experience says that it takes quite a bit of difference for a sustained time.