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Old 10-11-2012, 09:23 AM   #1
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unique and specific tire question

sorry for starting a new tire thread... but theres so much info currently out there its hard to find an opinion specific to my situation.

I have a 97 Sahara 4.0 sitting on what looks like a 2" body lift.
D30 front and D35 rear stock 3.55 gear ratio
I have a set of American Racing Atlas rims R15 I measured across and they look like they are 9.5" or 10" but depending on where you are actually supposed to measure they may be 8" wide (probably are 8" based on my current tire size and available sizes from AR)

When this set of tires wear out I want to move from the All Terrain to the Mud Terrain KM2's and my question is about size.

Im pretty sure I could get away with going from 31's to 32's with ease but I'm wondering if I could get away with going to 33's

my current setup
I am currently running 31x10.5 R15 BFG All Terrains with no issues, its nice and peppy all gears are fine.
I don't have a need for 1st gear on road, even with hills and can easily hit 80-85 on the highway.

my main concerns:
I think my shift rail is wearing out which makes 1st gear pop out every once in a while when im not in 4wd so I want to make sure the new tire size I'snt going to give me too much of a loss or be too hard for 2nd gear starts (I will fix the tranny but.. its still a concern of mine)

I want to make sure im not losing 5th gear usefulness for when I do take trips that involve the highway

I also want to make sure I'm not going to kill my axles with the bigger tires

Here are some tire specs:
BFG All Terrain 31x10.5 R15
weight: 42lbs diameter: 30.7"

BFG Mud Terrain 31x10.5 R15
weight: 45lbs diameter: 30.8"

BFG Mud Terrain 32x11.5 R15
weight: 52lbs diameter: 31.8"

BFG Mud Terrain 33x10.5 R15
weight: 49lbs diameter: 32.8"

BFG Mud Terrain 33x12.5 R15
weight: 58lbs diameter: 32.8"

based on weight I'm guessing I could get away with the 33x10.5, but aesthetically speaking, I might like the 32x11.5 more? the inch in width may look cooler than the inch in height, lol (real important I know)

thoughts, opinions, pictures, all are welcome.

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Old 10-11-2012, 10:37 AM   #2
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Ive never heard of 3.55 being stock gearing for a TJ. They either came with 3.07 or 3.73 or if you had a rubicon 4.10. 33's with a D35 is pushing the limits. If you dont off road much then you should be ok. As for lift. I would add a 2" BB.

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Old 10-11-2012, 12:22 PM   #3
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3.55 was indeed one of the ratios available for '97 TJs which did not get 3.73 as a choice until 1998.

Obidya, a 2" body lift lifts the floor pan 2" further up from the transmission which can restrict the shifter throw enough that it cannot fully engage 1st gear. That can make it prone to pop out of 1st gear. I would temporarily remove the shifter boot to see if that reduces the restriction enough to let the shifter more fully engage 1st gear. The usual fix for this is to move the console forward a tad, some get results by replacing the stiff rubber OE shifter with a more flexible leather shifter boot.

Or get rid of the 2" body lift.
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Old 10-11-2012, 06:15 PM   #4
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Quote:
Originally Posted by Jerry Bransford View Post
3.55 was indeed one of the ratios available for '97 TJs which did not get 3.73 as a choice until 1998.
^ true, and I didnt know that until I got my build sheet from Jeep

Quote:
Originally Posted by Jerry Bransford View Post
Obidya, a 2" body lift lifts the floor pan 2" further up from the transmission which can restrict the shifter throw enough that it cannot fully engage 1st gear. That can make it prone to pop out of 1st gear. I would temporarily remove the shifter boot to see if that reduces the restriction enough to let the shifter more fully engage 1st gear. The usual fix for this is to move the console forward a tad, some get results by replacing the stiff rubber OE shifter with a more flexible leather shifter boot.

Or get rid of the 2" body lift.
Get rid of its the overall plan

I have done all I can think to do. I replaced the shifter bushing, pulled out the center console and everything that came anywhere near the shifter, replaced both motor mounts and the transmission mount, did a fluid flush and change to redline MTL (holy cow what a difference)

I did move the console 1/2" forward when removing it didn't help, its now centered to the shifters movement

AX15 thread
If there is anything else I can try without tearing into the transmission I'm all ears. Im pretty much just planning on waiting for the time and money to tear it apart some weekend and replace everything related to 1st and 2nd gear

whats weird is... it seems to do it at random, and I cant seem to force it to happen, Im assuming the high sulfer fluid the PO used just wore out the syncros enough to effect 1st. Its the only thing that makes sense in my mind


but as far as tires are concerned, lol
from what I understand about tire size.... Its more the actual weight than it is the radial distance that matters so I'm guessing my best shots going to be with the 32's or the 33x10.5's I may research some other brands see if I can find anything within that weight class.
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Old 10-11-2012, 06:47 PM   #5
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I wouldn't think the tire size has anything to do with the transmission popping out of gear. Perhaps it's caused by nothing more than a bent shift fork inside the transmission not fully engaging the gear.
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Old 10-11-2012, 07:24 PM   #6
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Originally Posted by Jerry Bransford View Post
I wouldn't think the tire size has anything to do with the transmission popping out of gear. Perhaps it's caused by nothing more than a bent shift fork inside the transmission not fully engaging the gear.
Sorry for the confusion..
My tire question has nothing to do with the transmission, Im sure it is something like that, either a bent fork or a worn syncro. I am planning on opening it up at some point and fixing it.

I only brought up my transmission issue to point out that currently I like my ability to start in 2nd and am wondering how much a larger tire would impact that. Its likely Ill get new tires before I convince myself to break open the transmission.
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Old 10-11-2012, 07:32 PM   #7
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really.... the question is

how much of a drivability difference is there between a 31",32", and 33" tire
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Old 10-11-2012, 09:17 PM   #8
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duratrac 33's are pretty light even at 33 12.5
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Old 10-11-2012, 11:25 PM   #9
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Quote:
Its more the actual weight than it is the radial distance that matters

This is false.
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Old 10-12-2012, 07:31 PM   #10
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This is false.
yeah? being the nerd I am, I have been trying to find some math to back up the torque required so I can build a nice little calculator... But I have no idea what im doing lol
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Old 10-12-2012, 07:36 PM   #11
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This is false.
X2. The larger tire's diameter increases its radius distance from the axle shaft which increases how much leverage the tire has against the drivetrain. It's harder for the engine to overcome the bigger tire's larger diameter's longer lever against it in other words. Not to mention the larger diameter's tire produces a higher effective axle ratio against the engine which lowers the engine RPMs at any given speed so at the same speed, the lower engine rpms from a larger diameter tire causes the engine to generate less power at the same speed it would with a smaller diameter tire.
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Old 10-12-2012, 07:40 PM   #12
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ok, so if im understanding this correctly...
the bigger the radial distance the more initiating torque will be required, and the weight becomes a factor in the form or rotational inertia.. esentially harder to get moving easier to keep moving.

so with my setup.. I really dont want to go to the 33's with out some mods either to gear ratio or something more extreme, but the 32's should be ok.

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