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How to figure out my gear ratio?

ggraham

ggraham

Geoff Graham
City
Atlanta
State
GA
I can't find a gear ratio tag on either the front or rear cases in my 84 Scrambler (4.2L, 5T). I have an AMC 20 cover in the rear and a Dana 30 in the front. How can I figure this out? It's time for a new rear end, I want to match what I have in the front, and I don't want to order the wrong ratio!
 
Jack the Jeep up and rotate the tire by hand 1 revolution. Count how many times the driveshaft rotates. That will give you the ratio. For instance if the drive shaft rotates just a little over four times, you'll have a 4.10 rear gear. It helps to mark the drive shaft and the tire with some chalk.
 
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You could pull diff cover off and read numbers stamped on ring gear as well. You may have to divide the numbers depending on what they say. You could count the teeth of the ring gear and pinion and divide ring gear into pinion. designerRob's comment prior saves you diff fluid, but pulling cover would help confirm as well.
 
If the gears haven't been changed, you could look on the machined spot next to the cover and find the Letter/s stamped there and that indicates the gear ratio. Find the Letter and who ever sees your post first will look it up for you.
 
Thanks, y'all. Can't find any stamped letters and prefer not to pull the cover until I'm ready to do the swap. I'll go with the jack/rotate method. 🙏
 
Rotate the wheel and post up your estimated driveshaft rotations and people will let you know the official ratio.
 
A little more info from when jumped through the same process a while baclk.

X

Thread 'Axle Ratio?'

I’ve been banging my head on the floor of the shop and figured it was time to ask for expert help. I’ve got an 83, and checking the AMC 20 rear end for an axle code, it has a CC stamp. Checked the chart and read if as a 3:31 with a trac-lock. Then I realized I read the wrong chart, that I had looked at a NT chart. Found the right chart and for the WT, then it should be 4:10 with trac-lock. Unfortunately, from driving it, I was pretty confident they aren’t 4:10s.

Jacking up the vehicle, I got about 1.6-1.7 turns on the driveshaft from a single rotation of the wheel. I was expecting double...
 
yes, you have to multiply x2 on driveshaft/yoke turns. Plus unless locked leave one tire on the ground.
 
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