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Unread 03-28-2013, 10:08 AM   #16
ejschultz
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With open diffs, weather it's on the front or the rear, in 2 wheel drive, it shouldn't matter. Up front, it'll just work. It'll probably throw off the steering a little though. Out back, it'll become the drive wheel. It has to spin slightly faster to keep up with the non spare tire. This can throw off the speedometer. Even in full time with a 242, short distances shouldn't matter. I can see long term effects like stretching the chain if you drive it like that for an extended period of time. You would want to keep the transfer case unlocked though. You could damage it in part time with a smaller wheel up front or out back.

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Unread 03-28-2013, 11:54 AM   #17
MyKeyB
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Quote:
Originally Posted by AmishFury View Post
you people with factory full size spares disgust me... i had to go find a rim to get my full size spare
From the build sheet:
TBBP Full Size Spare Tire
TBLS Inside Mounted Spare Tire
TRDP P225/70R16 All Season Tires
TZAS Goodyear Brand Tires
WDNP 16X7.0 Aluminum Wheels
WKBP Matching Spare Wheel
WLZC All Aluminum Wheels

lol.
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Unread 03-28-2013, 03:35 PM   #18
Adam86
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Well l made the mistake of running my spare tyre on the left front for maybe 250 miles with the 249 tranny. Can't remember the sizes but the spare was about an inch smaller in diameter. I replaced the tyre but it was too late.

But what actually happened is l killed my front diff and propshaft not the tranny. The diff became noisy and clunky when turning and the prop failed causing the Jeep to violently shake. Couldnt drive above 15mph. Replaced both and it was back to normal.
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Unread 03-28-2013, 08:39 PM   #19
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Okay, let's review....

If you have the 249, all the tires must be the same size. If not, you'll ruin the transfercase.

If you have the 242, you'll still want the tires to be the same size yet, a slightly smaller or larger tire will work for a short period of time.

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Unread 03-29-2013, 12:03 AM   #20
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Quote:
Originally Posted by Texas ZJ1 View Post
Okay, let's review....

If you have the 249, all the tires must be the same size. If not, you'll ruin the transfercase.

If you have the 242, you'll still want the tires to be the same size yet, a slightly smaller or larger tire will work for a short period of time.

Hunter
this post should be added to the FAQs
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Unread 09-08-2014, 05:56 PM   #21
MrRoundel
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I'm glad that I searched before posting, as this answered my question, at least for the most part. My Jeep is a '98 4.0L w/Selectrac NP242 transfer-case with 180K miles on her. Well maintained and only off-road for a couple of trips to Baja. I never had to put it in 4 wheel drive.

My stock tires are 225/75/15 on my Jeep. I'm planning trip to Mex, where I drive a 60 mile dirt/rock road that is known to destroy tires. My spare is also a 225/75/15, and I'd like to keep it as the spare, or perhaps bring the best of my remaining tires to that position, while replacing the four running tires with either Goodrich TA all-terrains or Michelin A/T2's. In order to do so, or even with the Michelin LTX A/T2's, I must go one size up -- 235/75/15. For purposes of economy, I'd rather not buy 5 new TA's or A/T2's even.

In the event that the road eats a tire on the way in or out, how long is it safe to drive with the smaller spare? It would be highway driving on Highway One for about 350 miles to get back to the US border. Would that be doable, providing I stay in 2 wheel drive? If I have to go into 4 wheel on the dirt road on the way to the highway, should I stay out of "Part-time" positions? I doubt if I'd need them anyway, but one never knows, Maybe I'll have to pull a buddy out of the 12" moon dust, or? I'm also thinking that I could get a used 235/75/15 for spare duty. Many thanks in advance.

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Unread 09-09-2014, 04:04 AM   #22
ejschultz
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225/75/15 = 28.3" diameter
235/75/15 = 28.9" diameter

Take that for what it's worth. You're not looking at a huge difference in diameter. Keep in mind, however, the age and wear of the spare may make that difference bigger. My numbers are also from doing math. Different manufacturers are going to have slightly different diameters for tires listed as that size. Your best bet would be to actually research the tires you want and compare their specs to the tire you want to use for your spare and see what the difference actually is.
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Unread 09-09-2014, 09:11 AM   #23
MrRoundel
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Thanks, ejschultz. I too figured that the difference is about 1/2" between the tire sizes. I'm leaning towards going with the used spare of the proper size route, although it's really not saving me much. Once consideration is that the spare cover is pretty tight on the 225, and might have a tough time wrapping itself around the 235. It's always something...no perfect answer.

If I have to use the spare on the front, and keep the transfer-case in 2 wheel drive, will harm come to the transfer case? In other words, if the front differential is open, and I'm running power to rear wheels only, does it matter much on size, aside from steering issues? Bear in mind I don't have a great grasp of 4 wheel-drive terminologies. Thanks again.
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Unread 09-09-2014, 09:22 AM   #24
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The front diff, drive shaft, axles are always spinning. No matter what. Even in 2wd.
You would need locking hubs to disengage the front and no ZJ came like that.
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Unread 09-09-2014, 10:51 AM   #25
monte350
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the difference in size between the tires may only be 1/2" in diameter, I think the more important number is circumference. there is actually about 2 inches difference when you measure the circumference of the tire.
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Unread 09-09-2014, 10:52 AM   #26
ejschultz
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Since, when in 2 wheel drive, the transfer case has no physical connection between the front output and anything else, you'd be fine. The smaller tire would cause the front drive shaft to ultimately spin faster than the rear, but that won't cause any damage because it's not physically connected to the drive line at that point. Full time and part time 4 wheel drive would be a different story.
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Unread 09-09-2014, 10:55 AM   #27
MrRoundel
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I guess when I opted for the Selectrac, I was right for all the wrong reasons. I had figured that it was different from the Quadratrac, in I'd be running "true" 2 wheel drive when the selector was so selected. Shows what I know. I guess I'll be safe and at least get a spare of the proper size, whether new or used. Many thanks for everyone's input.
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Unread 09-09-2014, 10:56 AM   #28
ejschultz
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Quote:
Originally Posted by monte350 View Post
the difference in size between the tires may only be 1/2" in diameter, I think the more important number is circumference. there is actually about 2 inches difference when you measure the circumference of the tire.
This is correct. The number you really care about is revolutions per mile. Since revolutions per mile is directly correlated to the circumference of the wheel and that's directly correlated to the diameter of the wheel, any of the numbers work. The further you get into the math behind it, the more you will see how a small change in diameter will affect the overall revolutions per mile. A 28.3" overall diameter is about 750 revolutions per mile. A 28.9" tire is roughly 698 revolutions per mile.
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Unread 09-09-2014, 11:47 AM   #29
MrRoundel
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Quote:
Originally Posted by ejschultz View Post
Since, when in 2 wheel drive, the transfer case has no physical connection between the front output and anything else, you'd be fine. The smaller tire would cause the front drive shaft to ultimately spin faster than the rear, but that won't cause any damage because it's not physically connected to the drive line at that point. Full time and part time 4 wheel drive would be a different story.
Thanks, ejschultz. I missed your recent post as I was typing mine.

That's the answer I was hoping for. I certainly wouldn't plan on driving with the mixed sizes for long, I would only be in 2 wheel-drive, and it would just be for the border drive. Of course, that takes it for granted that it happens on the way out, and there's no telling on that one, is there? Worst case scenario is 700 miles with mixed sizes in 2 wheel drive. That's if I get the flat in TJ on the way south, which one doesn't want for a number of reasons. Cheers.
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