So I thought I had 3.73 gears

[Agreen]

So my 1998 (4.0, 2wd, Aw4) did not have an axle tag, so I've never really known the rear ratio. I converted it to 4WD in 2017 and instead of swapping the rear end I just counted the number of input vs output turns. I got about 3-3/4 turns on both axle yokes when the tires made a full revolution. Yes, I've had the rear differential cover off several times, but I've never once considered counting pinion/ring teeth

At 60 MPH the tach has always read 2000 rpm. I also verified this with GPS and the OBD scanner. Without any question, it's 2000 rpm and 60 MPH.

Using an RPM/Ratio/speed calculator I get 60 MPH when I input the following data:
3.73 axle ratio
28.3 inches tire height (225/75r15)
0.75 transmission ratio
2000 RPM

Fast forward to today. I put a 3" lift, 31" tires, and want to correct the speedo. Easy enough, the ratio is still fairly forgiving for 31" tires, so I'll just pop a 33 tooth speedo gear in, right?

So I pull the old gear out and it's a 34. Weird. I expected a 36. Determined to know the answer, I prepare to pop the front diff cover and count teeth. Then I noticed the tag (which I somehow missed before). Ok, so I have been a little lazy and only topped the front diff off when I did the swap. Anyway, there it is. It says 3.55 on it. Makes sense. A 34t speedometer is accurate for 3.55 ratio gears.

So what's the deal? If it's 3.55 I should be reading 63 mph at 2000 rpm, or 1900 rpm at 60. So now I have to order a different gear, and I'm still confused as to why everything else points to having 3.73 ratio when I most likely have a 3.55.

I'm really only asking because I don't want to order the wrong gear and still have the speedo off still.

 

[Tim_MN]

The obvious answer is pop the cover and count the teeth. AFAIK after Chrysler bought Jeep back in the day they no longer offered XJ AW-4 4.0L 3.73 gears.

 

[cruiser54]

The obvious answer is pop the cover and count the teeth. AFAIK after Chrysler bought Jeep back in the day they no longer offered XJ AW-4 4.0L 3.73 gears.

Correct.

 

[maxbraketorque]

So is your front diff a 3.73 and your rear diff a 3.55? If yes, I guess you've never driven on the street in 4WD mode?

 

[89Mock]

Take off the cover
Remove the ring gear
count the teeth on the pinion
count the teeth on the ring gear
Do this for both axles.
install new differential covers with lube locker gaskets and differential fluid.

 

[Black1990jeep]

So my 1998 (4.0, 2wd, Aw4) did not have an axle tag, so I've never really known the rear ratio. I converted it to 4WD in 2017 and instead of swapping the rear end I just counted the number of input vs output turns. I got about 3-3/4 turns on both axle yokes when the tires made a full revolution. Yes, I've had the rear differential cover off several times, but I've never once considered counting pinion/ring teeth

At 60 MPH the tach has always read 2000 rpm. I also verified this with GPS and the OBD scanner. Without any question, it's 2000 rpm and 60 MPH.

Using an RPM/Ratio/speed calculator I get 60 MPH when I input the following data:
3.73 axle ratio
28.3 inches tire height (225/75r15)
0.75 transmission ratio
2000 RPM

Fast forward to today. I put a 3" lift, 31" tires, and want to correct the speedo. Easy enough, the ratio is still fairly forgiving for 31" tires, so I'll just pop a 33 tooth speedo gear in, right?

So I pull the old gear out and it's a 34. Weird. I expected a 36. Determined to know the answer, I prepare to pop the front diff cover and count teeth. Then I noticed the tag (which I somehow missed before). Ok, so I have been a little lazy and only topped the front diff off when I did the swap. Anyway, there it is. It says 3.55 on it. Makes sense. A 34t speedometer is accurate for 3.55 ratio gears.

So what's the deal? If it's 3.55 I should be reading 63 mph at 2000 rpm, or 1900 rpm at 60. So now I have to order a different gear, and I'm still confused as to why everything else points to having 3.73 ratio when I most likely have a 3.55.

I'm really only asking because I don't want to order the wrong gear and still have the speedo off still.

the difference between these gears 3.55 and 3.73 is about 5%. if you are off ever so little in your axle or wheel rotation count, you will get the wrong answer. 5% is not much if you are eyeballing it.

As for the tach, again 5% might be in the error range of the tach calibration. Same for the speedo.

Also the tire diameter used in the calculations maybe off. 5% of a 28.3 inch tire is 0.71 inch on the radius. Allowing for tire pressure variations, tread wear, the effective radius of the tire at ground contact point may not be exactly 28.3 / 2 = 14.15 inch

All these things can add up to 5% error. a tach a little off maybe 2 percent, the tire radius off maybe a quarter inch (that is about a 2% error), the speedo a little off maybe 1 percent, maybe even some torque converter slip. You can easily explain a 5% error.

Now the tag is one thing but who knows if anyone swapped stuff and left the tag alone, right?

Count the teeth, I think you can probably figure it out without having to pull anymore than the cover. so it will be a fine time to change the oil. and add a drain plug, I drilled and taped for a pipe plug, I think it is 1/4 inch pipe thread. It dont need to be too big, drill thru the thick wide ring around the case at the lowest point. Tap deep enough so plug is fully recessed. I drilled the plug's end and epoxied a round magnet into the hole drilled into the end of the plug so now my diff has a drain AND a magnet filter!

Dont forget to do a counter bore or chamfer maybe 1/8 inch deep on the bottom side of the threaded drain hole, else a rock hit may bugger up the lowest couple threads if you leave the threads running all the way to the bottom surface.

The cast iron diff. case drills and taps very easily, piece of cake with simple tools to drill and tap the case for a drain plug. This will save you lots of time and gaskets with fluid changes. and the magnet is just plain goodness for the rotating bits inside.

Have fun.