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I have a question that was breifly touched on in another thread but I believe this deserves it's own thread beause of its potential importance.

So many of us are running larger tires and do not have a spare that is the same size. If you were to run the stock spare with three 255/70/17s (since 255/70/17 will not fit in the spare location with tow package) - Are you pretty much gauranteed to rip up the transfer case or would the ESP/traction control put it in limp mode and save damage from happening?

I need a way to carry a spare that looks OK. I dont care for that swinging tiregate that a couple guys have - It looks too generic and too far away from the tailgate - Get Lost 4x4 should design a hitch mounted one that sits up closer to the tailgate that looks stock. That's something I would buy - Hint, Hint :)
 

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I am not an expert by any means but I would bet $ that you will damage your XK by using a spare that is a different size for any real distance. Looking forward to our experts opinion.
 

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...I can't believe nobody has an opinion on this?
 

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If you drive more than 1" on mismatched tires your going to do damage. The NV245 is fully locked all the time so if you manage to even get it to move from the ESP working you will either tear up the differentials or you will kill the transfer case. If you are not running with a full size spare right now then you need to call for a tow if you get a flat tire. As for a solution to getting a spare somewhere on there, you can try something like this there are several manufacturers out there but this is just the first one I came across on google. http://www.sportsimportsltd.com/unhimosptica.html
 

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If you drive more than 1" on mismatched tires your going to do damage. The NV245 is fully locked all the time so if you manage to even get it to move from the ESP working you will either tear up the differentials or you will kill the transfer case. If you are not running with a full size spare right now then you need to call for a tow if you get a flat tire. As for a solution to getting a spare somewhere on there, you can try something like this there are several manufacturers out there but this is just the first one I came across on google. http://www.sportsimportsltd.com/unhimosptica.html

That carrier is an option but you can't open your liftgate then.

I guess I dont really understand the difference of having a smaller tire on one wheel (more revolutions) and off roading and having one tire spinning (losing traction) and the others not spinning.
 

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Driving with the original sized spare with 3 other oversized tires is obviously NOT a great idea, and not a long distance solution, but I dont think there would be major damage to the differentials, especially in 4H. You may have to shut the traction control off, due to different axle rotations, but nothing in the 4x4 system is 'locked up' thats the whole idea of the clutch packs in the axle difs and the trans case. Thats my thought at least, would like to hear from the techs/engineers on this.
 

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I'm with Bulldog9

I wouldn't do it at highway speeds of 70mph but to limp to a service station. In a turn one wheel travels faster than the other and it does not make the transfer case OR differentials explode

just my opinion though
 

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I'm with Bulldog9

I wouldn't do it at highway speeds of 70mph but to limp to a service station. In a turn one wheel travels faster than the other and it does not make the transfer case OR differentials explode

just my opinion though
x3,
I cannot see enouph difference in circumference to cause any harm.
I believe the transfer case would only interpit the slight driveshaft speed difference as cornering.
The driveshaft r.p.m. difference is the same with the mismatch, regardless of speed, so I wouldn't be concerned about operating speeds, beyond being consevative.
From a wear standpoint, the differential pinions would experience more potential than the transfer case as it uses planetary gears.
But, I'm talking thousands of miles with a mismatch, not the 50 or so miles before the correct tire is remounted.
If I was running the moderate oversize being discussed, I would have little concern if I used the original circumference as my spare.

I'm basing my opinion on:
1. Minimal difference relative to tire circumference.
2. The fact that the circunference difference must be divided by the axle ratio to determine the change in driveshaft R.P.M. front to rear.
3. The fact the transfer case is only locked in low range and would not be operating on hard ground in this mode.

My $.02, but I bet Jeep would agree.
.....Rob
 

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Driving with the original sized spare with 3 other oversized tires is obviously NOT a great idea, and not a long distance solution, but I dont think there would be major damage to the differentials, especially in 4H. You may have to shut the traction control off, due to different axle rotations, but nothing in the 4x4 system is 'locked up' thats the whole idea of the clutch packs in the axle difs and the trans case. Thats my thought at least, would like to hear from the techs/engineers on this.

There are not clutch packs in the differentials on QT I and QT II. They have open differentials (spider gears)!
 

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All three of you are wrong and apperently know very little about the driveline on a Full Time Four Wheel Drive Vehicle. You never want to use a different size tire especially having a spare tire size of 245/65R17 and the other three tires being 255/70R17 (or whatever size tire that was mentioned earlier in the thread). The stock tires are 29.5 inches tall and a 255/70R17 is 31.1 inches tall, thats a huge difference. You should always have a full size spare with you.
 

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All three of you are wrong and apperently know very little about the driveline on a Full Time Four Wheel Drive Vehicle. You never want to use a different size tire especially having a spare tire size of 245/65R17 and the other three tires being 255/70R17 (or whatever size tire that was mentioned earlier in the thread). The stock tires are 29.5 inches tall and a 255/70R17 is 31.1 inches tall, thats a huge difference. You should always have a full size spare with you.
I generally cannot be cohearsd into a pi$$ing match, but you have gotten a bit close.
I must respectfully disagree with you.
Heres why,
1. From the Jeep homepage, engineering FAQ, I quote: Full time 4wd ALLOWS different driveshaft speeds.
2. Huge tire difference eh? Remember, we are discussing circumference here, not tire height. The circumference of a 29.5 tire=92.676 in
The circumference of a 31.1 tire= 97.703 in
So... if I roll the 29.5 one full turn it travels 92.676 inch across the floor.
If I roll the 31.1 one full turn it travels 97.703 inches.
So now we know the larger tire will travel 5.027 inches further for every revolution.
Since the 92.676 tire is only on one side and the 97.703 is on the other, the pinions will reduce this difference by 1/2.
Therefore, the ring gear will only turn 2.51" further than the other ring gear for every wheel revolution.
Now, we introduce the axle ratio, which on the vehicle in question is a 3.73.
So, for every 3.73 turns of the driveshaft, the ring gear makes one turn.
Or, for every 3.73 turns the axle assy with mismatched tires goes 95.189 inches.
The axle with the matched tires goes 97.703 inches.
We will assume the smaller tire is on the rear.
The difference the transfer case sees: .67 further rotation of the rear driveshaft to complete one full wheel turn.

Does that really sound huge to a transfer case designed to allow different driveshaft speeds?
I stand by my original opinion,
Rob
 

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There are not clutch packs in the differentials on QT I and QT II. They have open differentials (spider gears)!
Roger, the clutch packs which could provide a lock are part of QD2, and provide or limit slip (side to side) that was my point and QT 1&2 make it even less of an issue, as they are open differentials, meaning you can spin one tire while the other barely turns at all.

It's the same as the tiny little donut you would put on the drive axle/wheel in 90% of cars on the road today, even those with limited slip/stability and traction control systems do so with no damage to the differential, maybe some ABS/TCS issues, but not mechanical unless it is a truye closed (locked) differential.

The system/differentials are designed to accomodate for this, for turns, etc. You could drive your Commander in a full steering wheel lock circle for a very long time and not destroy your differentials, even though they are turning at very different rates. Think about it. If you are in a full left hand lock, at least 3 if not all 4 tires are turning at different rates.

Now if you LOCK the differentials, that is a different story. We have 3 open differentials, side to side, front to back.

The open differentials and clutch packs (QT 1&2, QD1) provide enough play side to side - front to back to not make this a problem for the short trip to the tire shop, same as the temp donut wont damage your front/rear differential on 2WD vehicles.

Of course we all have our opinions, but I wont be buying 5 tires when I replace and buy oversize tires next year. I will however be able to buy a few more cases of my favorite beer and think of those of you who do when I drink one :rofl:
 

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I generally cannot be cohearsd into a pi$$ing match, but you have gotten a bit close.
I must respectfully disagree with you.
Heres why,
1. From the Jeep homepage, engineering FAQ, I quote: Full time 4wd ALLOWS different driveshaft speeds.
2. Huge tire difference eh? Remember, we are discussing circumference here, not tire height. The circumference of a 29.5 tire=92.676 in
The circumference of a 31.1 tire= 97.703 in
So... if I roll the 29.5 one full turn it travels 92.676 inch across the floor.
If I roll the 31.1 one full turn it travels 97.703 inches.
So now we know the larger tire will travel 5.027 inches further for every revolution.
Since the 92.676 tire is only on one side and the 97.703 is on the other, the pinions will reduce this difference by 1/2.
Therefore, the ring gear will only turn 2.51" further than the other ring gear for every wheel revolution.
Now, we introduce the axle ratio, which on the vehicle in question is a 3.73.
So, for every 3.73 turns of the driveshaft, the ring gear makes one turn.
Or, for every 3.73 turns the axle assy with mismatched tires goes 95.189 inches.
The axle with the matched tires goes 97.703 inches.
We will assume the smaller tire is on the rear.
The difference the transfer case sees: .67 further rotation of the rear driveshaft to complete one full wheel turn.

Does that really sound huge to a transfer case designed to allow different driveshaft speeds?
I stand by my original opinion,
Rob
Hope you used a calculator for all that figuring, just looking at the numbers gave me a headache :banana_hitit: Dang TBI............:cowfrown:
 

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Well outside of the army im a mechanic and i have seen differentials and transfer cases blow many times due to ignorant people throwing different size tires on their vehicles.

I guess what it all comes down to is that it is your vehicle and do what you want with it. You asked for advice, I gave an answer based on my experience.
 

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The difference the transfer case sees: .67 further rotation of the rear driveshaft to complete one full wheel turn.

Does that really sound huge to a transfer case designed to allow different driveshaft speeds?
I stand by my original opinion,
Rob

You would be suprised how much that little bit effects the transfer case.
 

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Since the 92.676 tire is only on one side and the 97.703 is on the other, the pinions will reduce this difference by 1/2.
Therefore, the ring gear will only turn 2.51" further than the other ring gear for every wheel revolution.
Rob
BTW each differential has one pinion and one ring gear. So I'm not sure where you get that one ring gear will turn 2.51" further then that other ring gear on the rear axle.
 

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Good discussion in here.

I'm with the opinion that says moving with different tire sizes on the same axle temporarily is not harmful, the speed difference due to turning is more that due to different tire size.

The NV245 is fully locked all the time
This is wrong, NV245 gets locked only in 4low... you can even test it on hard surface.
 

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So if one ring gear will turn 2.51" more per wheel revolution, just think how many revolutions the wheel will do at even a low speed of 20 mph. Do the math for that and get back to me and tell me it wont effect the drive line.

The first vehicle I got when i turned 16 was a 1987 Ford Ranger 2.9 Liter 4x4 Automatic. My brother had purchased it from our friends parents who decided to throw some larger tires on the front that were given to them and leave the stock tires on the rear (needed new tires for winter). It had snowed so they put the vehicle in 4x4 and lets say that they didnt even make it out of the drive way before blowing the transfer case. But that's not it. After purchasing it from them we tore down the transfer case and rebuilt it only to find out the tranny had blown to. BTW both front and rear axles has open differentials.

I realize this is different then running one tire a different size then the rest, but i just wanted to share the story with you guys.
 
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