Best way to figure ratio?

[barillms]

When looking at a Jeep to buy, if the tag isn't readable what's the best way to figure out the axle gear ratio without taking off the diff cover?

[5-90]

BEST way - take of the cover, count pinion teeth & ring gear teeth, divide ring gear by pinion.

Second best way - jack up either end, turn the driveshaft until the wheels go around EXACTLY once, and count driveshaft turns.

Alternate method - check for an ID tag retained by the cover bolts. It will have the Dana BoM number, a few other numbers, and a ratio on it. It will also tell you if you have Trak-Lok, and need to use LSD "Friction Modifier" for refills.


If the tag is missing, check the other end. I can't give you a fixed location for the rear tag, but the front will usually be on the passenger side of the cover (on the straight.) Both axles will be within a couple points of each other - otherwise, the drivetrain would grenade in 4WD.

Note that the tag does not apply if the gears hve been changed - and counting teeth requires removing the cover.

Typical ratios for 4.0/AW4 are 3.54/3.55, 4.0/MT 3.07/3.08, and four-cylinders seems to end up with 3.55, 3.73, or 4.10 gears. Early trailer tow packages seem to like 4.10 gears - but that's only up to 87-88-ish.

Good luck!

5-90

[barillms]

I imaging marking the tire and marking the DS would help you keep track of the rotations while you count. I was thinking of having a friend spin the tire one full rotation while I lay under and count the DS rotations. 3 3/4 DS rotations would be 3.73s right? While 3 1/2 DS rotations would be 3.55s?

[5-90]

Yeah - pretty close. Standards are, as I recall, 3.07, 3.55, 3.73, 4.10, and (maybe!) 4.55 ratios. Turning the driveshaft and watching the wheels will get you close enough to figure out what's going on - the "fine" count can come when you get the vehicle and do all your maintenance...

5-90

[carmike692000]

I never would have thought about counting rotations. That's a great idea!