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Old 04-22-2013, 12:13 AM   #1
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Calculating differential gear ratios?..

Is there a way to calculate the differential gear ratio by tire size and rpms at a particular speed?
I just got this Wrangler with 35'' Crushers. At 65mph my tach registers 2500 rpms. Speculations?
Thanks

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Old 04-22-2013, 12:17 AM   #2
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I'm too dumb to know how to post a link from my phone but if you google crawl ratio calculator it will bring up a few. I think its grimmjeeper that has exactly what you're looking for

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Old 04-22-2013, 02:22 AM   #3
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Old 04-22-2013, 03:52 AM   #4
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I might be missing it, but these only give a rough estimate because you cannot select what engine and transmission you have. I know the site you are thinking of and I will try to find it tomorrow if no one else finds it before then. It allows you to put in your final transmission ratio, gear ratio, and RPM to get your end speed.
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Old 04-22-2013, 04:29 AM   #5
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I might be missing it, but these only give a rough estimate because you cannot select what engine and transmission you have. I know the site you are thinking of and I will try to find it tomorrow if no one else finds it before then. It allows you to put in your final transmission ratio, gear ratio, and RPM to get your end speed.
If he is just trying to find his gear ratio this should get him close. Engine and trans shouldn't matter as the gear ratio will determine rpms at a given speed.

He could also jack it up and spin the tires and calculate approximate ratio by the number of turns of the DS.
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Old 04-22-2013, 06:54 AM   #6
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Safely jack the rea tires off the ground, mark or take note of the pinion position, immobilize one tire, rotate the other 2 full turns while counting pinion rotations. This can be done alone but is easier with a helper. If the pinion rotated just over 3 turns it's 3.07, 3 3/4=3.73, 4ish=4.10, 4 1/2=4.56, ect.
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Old 04-22-2013, 07:04 AM   #7
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If he is just trying to find his gear ratio this should get him close. Engine and trans shouldn't matter as the gear ratio will determine rpms at a given speed.

He could also jack it up and spin the tires and calculate approximate ratio by the number of turns of the DS.
The final drive ratio in the 5 speeds, 6 speeds, and autos are all different and do effect engine RPM.
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Old 04-22-2013, 10:15 AM   #8
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Is there a way to calculate the differential gear ratio by tire size and rpms at a particular speed?
I just got this Wrangler with 35'' Crushers. At 65mph my tach registers 2500 rpms. Speculations?
Thanks
Use this link: Gear Ratio Calculator

It allows you to enter in Transmission, Transfer Case, Tire Size, and Gear Ratio.

You can check different combinations that way to see what may/may not work best for you.
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Old 04-22-2013, 10:19 AM   #9
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Originally Posted by All Terrain TJ View Post
Safely jack the rea tires off the ground, mark or take note of the pinion position, immobilize one tire, rotate the other 2 full turns while counting pinion (driveshaft) rotations. This can be done alone but is easier with a helper. If the pinion rotated just over 3 turns it's 3.07, 3 3/4=3.73, 4ish=4.10, 4 1/2=4.56, ect.
That right there, or tell us what transmission you have & we can figure it out.
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Old 04-23-2013, 01:39 AM   #10
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Thanks guys for the great responses.. When I input 35' tires and 2500 rpm at 65mph the calculator had me right between 3.91 and 4.11..
Considering the room for error between the tach and speedometer I can't be sure which..
As Bransford requested (and thanks) the tranny is the automatic 3-speed for 2001..which of course they kept til 2003..jeesh, what were they thinking??
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Old 04-24-2013, 08:02 AM   #11
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Thanks guys for the great responses.. When I input 35' tires and 2500 rpm at 65mph the calculator had me right between 3.91 and 4.11..
Considering the room for error between the tach and speedometer I can't be sure which..
As Bransford requested (and thanks) the tranny is the automatic 3-speed for 2001..which of course they kept til 2003..jeesh, what were they thinking??
Using GrimJeepers Calculator (link I provided), 35" tires with your trans/trans case would give you 2558 RPM's at 65 MPH.

I think you have 4.10's.
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Old 02-16-2014, 03:16 PM   #12
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Originally Posted by All Terrain TJ View Post
Safely jack the rea tires off the ground, mark or take note of the pinion position, immobilize one tire, rotate the other 2 full turns while counting pinion rotations. This can be done alone but is easier with a helper. If the pinion rotated just over 3 turns it's 3.07, 3 3/4=3.73, 4ish=4.10, 4 1/2=4.56, ect.
I just did this today. My marked driveshaft rotated between 3 1/3-3 1/2. 2011 Auto should be a 3.73 I don't believe there is a lower factory option?
Is there a 3.23?
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Old 02-16-2014, 08:14 PM   #13
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never mind, I just saw that they came standard with 3.21's and 3.73 as an option. sucks for me

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