I picked up an '04 Rubicon with lots of after-market suspension parts. The differential gear tag still says the factory 4.10, but I am guessing it has been re-geared. Currently has 35s on it. I am thinking I could run it in a particular gear at a certain RPM and measure the speed via GPS, and come up with the gearing ratio. Is that flawed logic? Is there a chart or calculator somewhere that I can use? It is a 42RLE btw.
Unsure of my gear ratio
- Thread starter jwtj02
- Start date
James9100
TJ Enthusiast
With the caveat that both rear tires need to spin at the same rate. Ie,engage the locker first.i like to put tape on fender and tire,pinion and housing as a reference
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Checking GPS speed and RPM in a certain gear is a fine way to do it. You can experiment with calculations here. Actual tire sizes are usually smaller than the stated size: for 35s, try 34 to 34.5. http://www.grimmjeeper.com/gears.html
James9100
TJ Enthusiast
You don’t have to take anything off.
You’re watching the driveshaft not the gears.
Jack up the rear end with a floor jack, rotate the tire by hand, and watch the rear driveshaft under the Jeep, count the rotations as you rotate the tire one full rotation.
Edit:
Turn the tire 2 complete rotations if your other tire remains stationary and does not rotate. Turn the tire 1 complete rotation if the other tire turns in the same direction.
(Pulled from How do I determine my axle gear ratio?)
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I'd be surprised if it had anything other than 4.10's, otherwise the seller would have mentioned it.
The backstory is tragic, the prior-previous owner died unexpectedly. A friend of his purchased it from the family to help with expenses, and after a couple of years, he said he didn't get the whole jeep-thing... this guy really had zero knowledge about anything with the TJ. The TerraFlex long arm kit is immediately noticeable, and when I asked about it, all he could tell me was that it had a lift kit.
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