How important is an exact match spare? - Jeep Commander Forums: Jeep Commander Forum
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post #1 of 20 (permalink) Old 06-12-2010, 12:54 AM Thread Starter
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How important is an exact match spare?

So I am planning on attending the West Coast Camp Commander (as long as the work gods cooperate). Since my last major 4X4 adventure I have added some mods, and as I am sure many of you can relate to, ran out of funds.

I was looking for a good way to mount a spare, still use my enclosed roof box, and tow my travel trailer. This did away with my thoughts of throwing the spare in a roof basket, or using a hitch mount spare tire carrier.

I measured the old stock tire mounted in the factory position under the rear and found it to be 29" in diameter. My brand new tires post-lift are 32". Since I am currently out of funds, and still in need of a great alternative form of transportation for a full sized spare, I came up with the thought that maybe, just maybe, 29" is close enough to full size for the purposes of a spare.

I haven't had a full time 4X4 before, so I am not sure how that kind of drive can be affected by a couple inch difference. I am sure someone out there is the type of person who likes math waaaaaaay more than I do (doesn't take much for that to happen) and will know something about how many rotations this difference becomes, how that affects the gearing, etc, etc. Those are just the potential things I could come up with, wouldn't suprise me if there are more.

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2007 XK V8 4.7L Flex Fuel // Rocky Road 2.25 lift // Rocky Road Super Sliders // Mikey Thompson 33" tires // 4XGaurd Front Gaurd // 4XGaurd Belly Gaurd // 4XGaurd Matrix-HC with optional front hitch // Warn XD9000I hitch mounted winch // Graco Carseat for the Youngun


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post #2 of 20 (permalink) Old 06-12-2010, 08:30 AM
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The Math

I'll let someone else tell you about the mechanical aspects, but here's the math.

But first, the stock tire is 29.5 inches so I'll use that length instead of 29".

Pi * diameter = circumference
Pi is approximately 3.14159

A 32 inch tire will travel 100.53 inches on the ground in one revolution.
A 29.5 inch tire will travel 92.68 inches on the ground in one revolution.

That's roughly an 8% difference.

I don't know if the differentials can handle that or not, but at most, I would only use it to limp back to get a flat repaired or buy a new full size tire.

Do you have QT II or QD II 4x4 system?

Edit to add: If you have QTII, I think you would be ok. I think they have open axle diff's and if so, then turn the traction control off. And keep it in HI range. Hopefully, more knowledgeable people will be along to post shortly and correct me if necessary.

Last edited by GPintheMitten; 06-12-2010 at 09:41 AM.
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post #3 of 20 (permalink) Old 06-12-2010, 09:47 AM
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Here's another thought, if you had a problem on the trail, the 32" tire is pretty popular and I bet someone would loan you a spare until you get back to town to get a new one or repair.

Of course if it happened on the road, your on your own.

And if you have QDII with Electronic Limited Slip Diff's, I don't think I would do it. I bought a used 32 inch tire from Discount and had that for a spare for a few months. It cost about $45. Of course, that doesn't solve your stowing problem. I mounted my spare on the roof rack.
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post #4 of 20 (permalink) Old 06-12-2010, 09:47 AM
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In talking to a local transmission shop in town, he warned me against doing that. He said you will tear the drivetrain (especially the front diff) up rather quickly running mis-matched tires.

If you have a 2 wheel drive or if we had the ability to take the Commanders out of 4 wheel drive you could get by with it but since they are a full time four wheel drive and it electronically shifts torque around between the 4 wheels, you will run into issues.

I'm sure someone else will be along here that is more knowledgeable than I am but I figured I'd pass on what I've been told.
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post #5 of 20 (permalink) Old 06-12-2010, 11:33 AM
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Up to what size tire can the spare wheel well hold?
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post #6 of 20 (permalink) Old 06-12-2010, 05:21 PM
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Originally Posted by mikenaf123 View Post
Up to what size tire can the spare wheel well hold?
245/75/17 fits without a hitch receiver from personal experience. Any taller and you may have issues... with or without a hitch.
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post #7 of 20 (permalink) Old 06-12-2010, 08:53 PM Thread Starter
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Wheel hole

My 29" inch spare (might be a 29.5 tire as stated by GP, but actual tape measured diameter on my particular tire is 29" - worn maybe?) leaves only about 3/4" clearance on one side and about 1/2" clearance on another fitting into the underbelly recess. I looked at the items a larger tire would be rubbing and there is no such easy fix as a little grinding or bending either.

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2007 XK V8 4.7L Flex Fuel // Rocky Road 2.25 lift // Rocky Road Super Sliders // Mikey Thompson 33" tires // 4XGaurd Front Gaurd // 4XGaurd Belly Gaurd // 4XGaurd Matrix-HC with optional front hitch // Warn XD9000I hitch mounted winch // Graco Carseat for the Youngun


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post #8 of 20 (permalink) Old 06-12-2010, 09:12 PM
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Consider doing what I did...my 245/75 (about 31.5" I think) fits underneath with a hitch after deflating the tire a bit and slightly pushing it in around the hitch. It's not deformed enough to cause any damage to the tire, and when I need it, I'll inflate it with the $25 "Slime" portable air pump I purchased. Stormtrooper has fit a 265/70 in the stock spare location as well, although he trimmed the back of the hitch receiver as well. Good luck.
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post #9 of 20 (permalink) Old 06-13-2010, 02:15 AM Thread Starter
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I like this idea as well. Any Techies out there have any problems with this solution?

The best things in life can not be found on a map; they can only be found while Wandering.

2007 XK V8 4.7L Flex Fuel // Rocky Road 2.25 lift // Rocky Road Super Sliders // Mikey Thompson 33" tires // 4XGaurd Front Gaurd // 4XGaurd Belly Gaurd // 4XGaurd Matrix-HC with optional front hitch // Warn XD9000I hitch mounted winch // Graco Carseat for the Youngun


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post #10 of 20 (permalink) Old 06-13-2010, 10:38 AM
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Deflating a bigger spare is an excellent solution. You can even use some cinching ratchets to really squeeze it down if necessary.

For what it's worth, I personally have QDII and run with an undersized spare. I've had assurances from a Jeep engineer involved in designing the system that I'd be fine for limping off the trail like this until I could get my full-size rubber repaired. Not having a big ol' 5th tire saves me a lot of cash and room, not to mention I'm carrying less weight -- especially if getting a full-sizer means getting more steel in the form of carriers. Blech. Lighter is always better.

Besides, in my decades of Jeeping I have never had a flat on the trail. Nor can I recall ever having been with someone who did. One likes to be ready for a rainy day, of course, but not at much cost!
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