How do I tell my gear ratio?

How do I tell my gear ratio on my 61 stock cj5? Is there a differential tag?

 

There should be a tag under the cover bolts on both the front and rear axles.

Lacking that, the tooth counts are stamped into the ring gear, ie "47 11" is 47/11 = 4.27

 

Or, if you don't want to open up the diffs, you put the rear end of the jeep (or the whole jeep) up on jack-stands, mark with chalk or paint a spot on the driveline and its corresponding spot on the diff, plus a chalk mark on the tire lined up with the drive-shaft. now count the number of times the drive-shaft rotates when you turn the tire through 360 degrees. three and three-quarters turns of the drive shaft compared to one turn of the tire means you have 3.75 gearing. More likely 3.73 is the number. In my case, the shaft turns 5 and one-third plus a tad times to one turn of the tire. I am running 5.38 differential gears.

However, if you think you need the gear oil changed, then do it Timgr's way.