Hi. My buddy just bought a 07 auto jk. It has km2 35's and auto trans. Is there a way of telling what size gears he has by rpm's? He said he was cruising at 68 mph ( gps) and just under 2k rpm's. Thanks in advance everyone

 

With an 07 auto, there's a good chance he has 4.10's

 

Jack up the back tires and mark the driveshaft and a tire. If it's a little I've 4 turns of the shaft to one turn of the tire, 4.10s, little under four, 3.73, and so on

 

If you look at the rear differential there's a metal tag on the left side. There are a bunch of #'s on it, the last 3 is your gear ratio

if the tag is missing - use the method described by JKRed

 

Looks like they're 4.10's:

 

Yup. That's what it looks like. Thanks for the info

 

I don't think anyone has seen a tag on a JK differential.

Here's a way to tell by jacking one tire off the ground. If you jack both, it's hard to make sure both tires turn equally.

Chock the front tires, fore and aft.
Put the transmission in neutral.
Emergency brake off.
Jack the left rear tire off the ground.
Put a jack stand under the frame, just in case the jack fails (had to add this).
Put an index mark on the aft end of the drive shaft, visible from the left rear wheel.
Put an index mark on the left rear tire at the 6 o'clock position.
Rotate the left rear tire 2 complete revolutions while counting how many times the index mark on the drive shaft passes its original position.
That number will be your axle ratio.
Remove jack stand and lower jack.
Put transmission in gear or Park, apply emergency brake.
Remove chocks.
If you have limited slip, jack up both rear wheels and rotate one revolution, making sure both tires rotate equally.

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 4½ turns, you have 4.56
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

 

Juat call the dealership with the vin#. Done

 

Not a sure thing on a used vehicle.
Even a new vehicle build sheet can be incorrect.
For those who like to get their hands on their Jeep, jacking/spinning a tire is quicker.

 

This is an excellent explanation, but don't forget 4.5 turns equals 4.56