Jeep Wrangler TJ Performance PCM Tuning

Gearing, tire size, and RPM

ChrisTJ98

ChrisTJ98

TJ Enthusiast
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Joined
Feb 19, 2019
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147
Location
Louisiana
I’ve read a lot of past reviews. On this forum and others. I have a 1998 TJ manual with ax-15 trans 4.0 engine currently at dry rotted 31x10.50x15 and 3.07 gears. I want to regear a dana 44 I picked up. I am debating if I will stay with 31’s or go with 32’s. This is a drive around town maybe some road trips. Very little off-road. I want to get some power back and hopefully a little better mpg as well. I would like to hear from people actually running 32’s and 4.10 or 32’s and 3.73 gears. What is your rpm in 5 th gear at say 55 mph. I currently have H&R springs and a 1” body lift. I believe 32’s will fit if I replace rims. I was thinking bfg all terrains but share your brand of tire as well.
I want to hear from people actually running these gears/tires not what you have heard.
Oh and I live in south east Louisiana it’s very flat.
Thanks
 
ChrisTJ98 said:
I want to hear from people actually running these gears/tires not what you have heard.
Click to expand...
You want 4.10s with 31s/32s not 3.73s. I'm not running those gears/tires but I ran them for 10 years. I had the OE gears and tire size of 4.10s and 30.5" tires. Go to http://www.grimmjeeper.com/gears.html and plug your info in (take 1" off the tire diameter) and it will tell you what your RPM will be in any gear at any speed.
 
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L J said:
You want 4.10s with 31s/32s not 3.73s. I'm not running those gears/tires but I ran them for 10 years. I had the OE gears and tire size of 4.10s and 30.5" tires. Go to http://www.grimmjeeper.com/gears.html and plug your info in (take 1" off the tire diameter) and it will tell you what your RPM will be in any gear at any speed.
Click to expand...

Thanks I’ll give it a shot an go drive mine and see what it’s at for comparison. I wish I could remember what size came with my jeep. Seems like 28 or 29 inch tires.
 
ChrisTJ98 said:
Thanks I’ll give it a shot an go drive mine and see what it’s at for comparison. I wish I could remember what size came with my jeep. Seems like 28 or 29 inch tires.
Click to expand...

measure from the ground to the center of the hub/wheel and double it. Enter that number as the tire size for accurate info from grimmjeeper
 
going by actual tire size from manufacture I would say Grimmjeeper is about 100-200 rpms off which isn't bad.
 
I’ve got the 4.0 5 speed 32 inch tires with 4.56 gears. I think it’s perfect. Manufacturers actual tire size is still not actual size when mounted on your Jeep. My BFG KM3’s probably measure just over 31 inches
 
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Just went to the garage and measured 15.5 inches from center of hub to the floor. That’s on 32/11.5/15 KM3’s with 3000 miles on them. Running stock canyons with 26 PSI. I have corrected speedometer and turn about 2900 rpm at 70 mph
 
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GPK03X said:
Just went to the garage and measured 15.5 inches from center of hub to the floor. That’s on 32/11.5/15 KM3’s with 3000 miles on them. Running stock canyons with 26 PSI. I have corrected speedometer and turn about 2900 rpm at 70 mph
Click to expand...

I’m going by actual
Manufacture tire diameter which is a inch off. The 32 inch tire you’re running is it on a 15x8 rim? I know my rims need replacing as of years ago the tire guy couldn’t get one to balance so he used the spare tire rim. Does it really matter that much with amount of air? I run about 40 psi in my 31’s
 
ChrisTJ98 said:
I’m going by actual
Manufacture tire diameter which is a inch off. The 32 inch tire you’re running is it on a 15x8 rim? I know my rims need replacing as of years ago the tire guy couldn’t get one to balance so he used the spare tire rim. Does it really matter that much with amount of air? I run about 40 psi in my 31’s
Click to expand...

Yes 15 by 8 stock canyons
 
GPK03X said:
I’ve got the 4.0 5 speed 32 inch tires with 4.56 gears. I think it’s perfect. Manufacturers actual tire size is still not actual size when mounted on your Jeep. My BFG KM3’s probably measure just over 31 inches
Click to expand...

Same here, 4.0 5 speed with 4.56 gears and 32's and think it's perfect.
 
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ChrisTJ98 said:
I’m going by actual
Manufacture tire diameter which is a inch off. The 32 inch tire you’re running is it on a 15x8 rim? I know my rims need replacing as of years ago the tire guy couldn’t get one to balance so he used the spare tire rim. Does it really matter that much with amount of air? I run about 40 psi in my 31’s
Click to expand...

The forum experts recommended about 28 PSI for my 32’s. I think 40 psi is a little too much for 31’s
 
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ChrisTJ98 said:
I run about 40 psi in my 31’s
Click to expand...
200.gif
 
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GPK03X said:
Yes 15 by 8 stock canyons
Click to expand...

Thanks can you tell me what kinda lift you have? I put H&R springs in and was amazed how much lift I got and have a 1 inch body lift on it, it sure looks like it could clear 32’s. Ran 40 psi because god it’s such a slug to drive. It needs gears bad.
 
ChrisTJ98 said:
Thanks can you tell me what kinda lift you have? I put H&R springs in and was amazed how much lift I got and have a 1 inch body lift on it, it sure looks like it could clear 32’s. Ran 40 psi because god it’s such a slug to drive. It needs gears bad.
Click to expand...

I’ve got 2 inch spring lift with a 1/2 body lift. Also have 1 1/2 inch extended bump stops
 

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