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Old 12-26-2010, 07:44 PM   #1
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Spare tire need to match size of other four?

OK, what got me thinking about this is three things.

First, running 35's on the TJ, hanging a 5th 35" off the rear (say on an aftermarket heavy bumper with tire mount), that's a lot of extra weight flailing away back there all the time.

Second, last summer I came across a guy who was on 37's and broke a wheel badly, it wouldn't roll. He had no spare at all. We loaned him a 35", so he was on three 37's and one 35. He was able to drive several miles back to camp just fine.

Third, you know on many compact cars they have that little undersized spare tire and you occasionally see a car rolling along with one tiny spare and they get you home as long as you don't exceed 50 mph or something.

So I'm thinking about just carrying a standard size spare, not a 35". Cost is not a factor, it's just the extra weight back there that I'm thinking about. I have no shame, I don't care what it looks like. Anybody else just carry a smaller spare when running 35's or 37's?

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Old 12-26-2010, 07:48 PM   #2
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those undersized wheels are under size in width not circumference, so unless you want one corner sagging I suggest you get same size.

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Old 12-26-2010, 08:31 PM   #3
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my pontiac vibe's factory spare tire is much smaller in circumference. Id imagine running a slightly smaller tire is fine in emergency situations to get home would be fine and nothing the suspension and differential could not handle. Im sure you can only go so much smaller before the tire is not big enough to keep all 4 on the ground. I wouldn't go much more then a few inches smaller.

But again, id only do this as a last effort to make it home or to nearest tire store.

Im no pro though, just keep it in 2WD, putting it in 4WD or having the axle locked (if you have lockers) would be bad.
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Old 12-26-2010, 08:38 PM   #4
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I run 33's and my spare is a 31.
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Old 12-26-2010, 08:49 PM   #5
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I run 35"s and carried a smaller spare, until I had to use it. Blew a sidewall in Palo Dura cyn on a Sunday. Nothing open for miles. Drive 400+ miles to Albuquerque on one 33" tire. Not fun, really. I can't imagine running an even smaller spare, and the weight saved from a 35" to a 33" wouldn't be enough to make me do it again.

I recommend getting a matching spare and a suitable way to mount it.
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Old 12-26-2010, 08:55 PM   #6
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Ya know...it just depends on how bad of a jam you're in. I have ridden my four wheeler with a flat rear tire for about 6 miles. All I did was place an ice chest on the opposite corner and fill it with water. I see no reason that a 235 75 15 wouldn't allow you to limp home. Granted I wouldnt want to be wailing down the highway like that.
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Old 12-26-2010, 08:57 PM   #7
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From what I have heard, it is not good for the diff running a small diameter tire when the other is larger. For a short distance, ok...but not very far. I would stay with the proper size spare, or at least VERY close to the correct size.
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Old 12-26-2010, 09:06 PM   #8
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its bad for the bearings to run smaller tires. If you think about it a smaller tire rotates more then a larger one.. three same size tires and 1 different size can throw it off.. those small donuts are just to get you to a place to get your actual tire fixed..
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Old 12-26-2010, 10:15 PM   #9
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One size under is just fine as long as it is more a quick fix, and you are taking it to the tire shop then or the next day. It will not mess anything up, if it is not at a very high rate of speed which I think you wouldn't because the Speedometer only goes up to 100. For example, I run 33's with a 31 spare tire
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Old 12-26-2010, 11:59 PM   #10
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Originally Posted by 2010BulletProof View Post
its bad for the bearings to run smaller tires. If you think about it a smaller tire rotates more then a larger one.. three same size tires and 1 different size can throw it off.. those small donuts are just to get you to a place to get your actual tire fixed..
So, by that logic, stock size tires are bad for your bearings???

So long as you'r eonly using the spare to make it somewhere to get the other tire fixed, a smaller (preferably not TOO much smaller) spare would be OK, just look kind of dorky. If you have lockers though, you may want to stick to a same-size spare.
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Old 12-27-2010, 12:24 AM   #11
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Quote:
Originally Posted by monkeee2002

So, by that logic, stock size tires are bad for your bearings???

So long as you'r eonly using the spare to make it somewhere to get the other tire fixed, a smaller (preferably not TOO much smaller) spare would be OK, just look kind of dorky. If you have lockers though, you may want to stick to a same-size spare.
He's talking about running a smaller tire with 3 larger tires......
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Old 12-27-2010, 12:34 AM   #12
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So, by that logic, stock size tires are bad for your bearings???
you missed the point

A smaller tire requires more revolutions to travel the same distance as a larger tire. Hence, why you lower the gear ratio for bigger tires to lower the rotation.

It's not bad for your bearings to travel at the same rot# when all four tires are stock size, but for every 1 rotation at 35" you might complete 1.15 rotation on a 31" (just an example). Something has to compensate for that difference, and it's going to be your axle components like your bearings. On a open differential, that's not a big deal for a few miles, but it will do damage your axles very quickly if not addressed.

I would imagine running two different size tires on a locked differential would be an absolutely NO! Because you're rotation is locked at the same number of rotations on both sides with each DS rotation, you're going to bust something in your differential with the load imbalance.

Overall, open, LSD, or locked your asking for trouble. Running at two different rotations will reek havoc on your differential because something has to compensate for that difference. On an open differential, the benefit is that more power will go to the smaller tire.

To address the OP, the weight issue is always discussed. Over 31" on the rear stock hatch and your asking for hinge bending. If you're going to run 35s you have to pay to play, and that means upgrading your tire carrier too to a rear bumper mounted tire carrier or just carrying it in the backseat like many people do. Most clubs won't even let you ride unless your carrying a full size matching spare tire. Plus, used tires are cheap enough to throw on and have just in case of a jam.

I'd recommend anyone whose looking to upgrade to 35s to consider EVERYTHING that's required before jumping into it.

- Upgraded rear axle (Ford 8.8+)
- Upgraded brakes
- Frame mounted tire carrier
- 4"+ lift
- 8" width wheels, proper backspacing
- Regearing

I'm looking to run 35s on mine, but I know that it's a process and I'm making sure everything is done right. So right now I'm currently working on my 8.8 axle with brake upgrades and then lift and get new wheels. 35s aren't going to be any fun on the trails when I'm sitting with a broken axle or no spare.
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Old 12-27-2010, 07:15 AM   #13
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Originally Posted by jasonwrangler View Post
those undersized wheels are under size in width not circumference, so unless you want one corner sagging I suggest you get same size.
Many are smaller in diameter. My Mustang has a smaller diameter spare.
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Old 12-27-2010, 09:26 AM   #14
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When you put a smaller tire on it changes the speed on one of the two axles feeding one diff. To your diff it's like going around in circles and the diff is compensating for the speed difference. The whole point of putting LSDs or a locker is to drive all axles straight at the same speed. A locker will come apart if you drive it that way on hard surface. If you want to save weight get a narrower tire but the same circumference. I run 33 12.50 15s but my spare is 33 10.50 15. Works fine because axles are traveling the same speed. Dont make the mistake of comparing what you do with a car to what can be done with a Jeep...expensive mistake.
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Old 12-27-2010, 10:37 AM   #15
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I like the look of huge tires on the back of Jeeps. I don't think the difference in weight is all that significant to risk having to run with different sized tires.
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Old 12-27-2010, 11:55 AM   #16
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you missed the point
Oops, my bad.


Quote:
Originally Posted by BLK00TJ View Post
I like the look of huge tires on the back of Jeeps. I don't think the difference in weight is all that significant to risk having to run with different sized tires.
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Old 12-27-2010, 12:00 PM   #17
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Well, after all that, I think I've thought of another reason to carry a matching full size spare. That being, if you're set up to carry a small spare, after you have a flat and put the small spare on the Jeep, then how do you carry out the big flat tire? It won't go on your mount because it's too big, so I guess you'd have to put it inside the vehicle or tie it up on the roof somehow. So I guess I'll set up to carry around a big spare after all.
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Old 12-27-2010, 02:06 PM   #18
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That being, if you're set up to carry a small spare, after you have a flat and put the small spare on the Jeep, then how do you carry out the big flat tire? It won't go on your mount because it's too big, .
Larger tires will fit on the stock spare mount. I have a 33x12.5 on mine.
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Old 12-28-2010, 09:27 AM   #19
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The safest way is to keep it in 2WD, make sure the rear two tires are matching and put the smaller one up front
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Old 12-28-2010, 09:58 AM   #20
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Since you are just looking at the weight issue I'd say not to bother with the smaller spare. Get a full sized spare and then you dont have to worry about limping it home you just go on with your day.
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Old 12-28-2010, 10:18 AM   #21
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How come running 1 smaller tire only damage your diff over a longer period of time? the was i think of it is that if you have 2 different size tires it would turn you're diff into shredded cheese. And i have open diffs. I run 33s with a stock 30' spare. But my 33s only measure 31.5 id be afraid to use the spare.
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Old 12-28-2010, 04:32 PM   #22
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If you're on the road and get a flat and only have a smaller spare, then you can put the smaller tire on the unlocked axle and drive safely - hopefully you don't have to go too far (some folks put a lock-right up front and keep their rear open, so you might end up putting your smaller spare on the rear). If you're off road, why would you want to be driving a Jeep with mismatched tires? 12.5 vs. 10.5 width difference is no big deal. It's the diameter that counts.

I strongly suggest to everyone with a Jeep - get a full-size spare and be done with it, and dont' expect others to bail you out. Some people carry a plug kit and figure they're "good-to-go" if they get a flat. They don't work if you've popped a bead or ripped a sidewall. And do you really want to be holding your group up while you try and repair a tire, or ask someone to go "back to camp and bring back my spare that's sitting on my trailer"? Or ask to borrow someone else's spare tire?

(now I'm just ranting). Flashback - Once I was with a group Jeeping, and one of the Jeeps got a flat and had no spare tire. So I loaned him mine; we couldn't leave him there, and he couldn't go anywhere until someone loaned him a spare tire (and my bolt pattern matched his). And the more we Jeeped, the madder I got. I was thinking "What am I, his spare tire b!tch"? Should I have packed a lunch for him too, or give him mine if he didn't? If he gets a hole in this one (my spare), is he going to replace my spare tire? (probably not, but he'd probably tell me he feels bad about it, and then I'd probably be made out to be the bad guy for insisting he buy me a new tire and pay for it to be mounted and balanced). And if I get a flat, what am I supposed to do? Ask for my spare back? I'm all for helping folks out on the trail, but there's certain things you should have/carry to be prepared for common problems. Like a full-sized spare (preferably matching with tread).

Ahhh..., I feel better now. I'm done ranting. What started me on this?

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