How do I check gear ratio?

[Tink48]

Last year I bought a 2009 JK X. It was already lifted with 35's. I've read that if you put larger tires on, you should re-gear. How do I check to see if it was done by the previous owner?

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[TerryMason]

This should do it:

1. Chock the front tires, fore and aft.
2. Put the transmission in neutral.
3. Emergency brake off.
4. Jack the left rear tire off the ground (If you have limited slip then you'll have to jack up both rear tires).
5. Put a jack stand under the frame, just in case the jack fails.
6. Put an index mark on the aft end of the drive shaft, visible from the left rear wheel.
7. Put an index mark on the left rear tire at the 6 o'clock position.
8. Rotate the left rear tire 2 complete revolutions while counting how many times the index mark on the drive shaft passes its original position.

If the drive shaft rotated almost 3 1/4 turns, you have 3.21
If the drive shaft rotated almost 3 3/4 turns, you have 3.73
If the drive shaft rotated a little over 4 turns, you have 4.10
If the drive shaft rotated a little under 5 turns, you have 4.88
If the drive shaft rotated a little over 5 turns, you have 5.13

 

[Bounty__Hunter]

Have you ever serviced the differentials? Ever checked their fluid level?

This would be a perfect time to pull the diff covers to service the differentials. You can pull the ratio from the numbers stamped along the outside of the ring gear.

 

[69jeepcj]

Terry, I thought that both tires had to be off the ground and the trany in high gear (4th,5th or drive for auto) for a 1.1 ratio. I also though that both rear tires needed to be turned at the same time, and you count the drive shaft turns to one tire rotation. Wouldnt be the first time I was wrong though.

 

[JPNinPA]

Typically there is a tag with the gear ratio stamped into it. The tag is located the on one of the upper bolts that hold the cover on.

 

[JohnnyO]

Terry, I thought that both tires had to be off the ground and the trany in high gear (4th,5th or drive for auto) for a 1.1 ratio. I also though that both rear tires needed to be turned at the same time, and you count the drive shaft turns to one tire rotation. Wouldnt be the first time I was wrong though.

In this case the transmission and gear doesn't matter because the tire is essentially powering the driveshaft and not the other way around. The transmission is in neutral. You are only looking at how many times the one revolution of the tire spins the driveshaft.
Although I also think you only spin the tire around once, not twice.

 

[Jester32]

If you only lift one tire then it needs to be twice as the spider gears are 2:1

 

[Tink48]

Thanks for the info. I'll check it out this week. One more question, what gear ratio should I have?


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[Bounty__Hunter]

Many options were available, check for a metal tag bolted to the diff cover. Post up any numbers you see on the diff tag.

 

[Crazyantix_TJ]

Just call any jeep dealer parts department and give them the VIN# and they'll give you the gear ratio for you mr jeep