How to tell what gears are in truck you bought second hand?

[1LoudJT]

Without tearing a truck apart, is there a way to know what gears are in a truck that was bought second hand?

 

[smritte]

Dana differentials have a tag on the cover bolt (assuming its still there). If you think the gears were replaced, you pull the cover and count the teeth. Ring gear divided by pinon. That's the best way. Most times the factory only has one gear option based on model. Look up what was available. An example is Rubicon Gas motor came with 4.10.

None Jeep
Option label on driver door/pillar or glove box.

 

[Scott Macon]

smritte what are you running in your rig?

 

[smritte]

4.10 for now. I have 4.88 to put in but am saving for new lockers first.
I generally prefer 4.88 on a 33" tire. This normally puts most vehicles slightly lower then stock range. This compensates for the extra weight we add on and puts the engine RPM back into its power band.

 

[Sid Clayton]

So with 38" tires, what would you be using?

 

[smritte]

Realistically you should be at least 5.13 for 37-38.
"If" the front diff was a dana 30, I wouldn't run lower than 4.88. 5.13 is as low as I would go with dana 44's. I'm basing this on aggressive driving. The lower the gear, the smaller the pinion. If your not driving aggressive then your good going lower.

The key is to put your rpm band back where it should be. We loose a ton of torque when we go with bigger tires.

Put some numbers into a gear ratio calculator like this one (found it on a random search). What it wont show is what is the strength limit of the differential.

 

[Sid Clayton]

Oh dang, missed your response. Thank you! Funny to think that I know people who are running stock gears with their 37-38 wheels, which are in the range of 3.73!