I'm trying to get my speedometer to read correctly and the gearing that I thought my jeep had was way off. The ID tag on both diffs are gone/unreadable. It currently has a 34 tooth speed gear and 61 mph (gps) reads 50 at the dash. I have a 33 tooth gear that 58 mph reads 50 on the dash too (better but not right). Trying to find the gear ratio, but I can't tell if I'm doing something wrong. 19.9 rotations at the driveshaft is equal to 13 rotations at the tire. The ratio can't be 1.53:1 so what am I doing wrong?

 

I too had a similar issue. I guess my RPM gauge is off a little because that's what I was using to determine that I had 3.73 gears. Turns out I was wrong, and it was actually 3.55.

So anyway, the right way to do it is to get an accurate RPM reading from a scan tool (like an ELM 327 scanner) and GPS speed. Do this in 3rd (auto) or 4th (manual). That way the transmission is at a 1:1 ratio. From there you can get a good idea of what the ratio is by putting the numbers in a ratio/speed calculator.

I use this one. Just keep changing the ratio number until you get the correct speed. Keep in mind, advertised tire sizes aren't actual. For instance, 31" tires are usually 30.5" (and so on)

https://spicerparts.com/calculators/...rpm-calculator

 

You also need to account for tire size. The right speedo gear can be found but you have to know what gearing you have and your tire size. Here is a link to a chart you can use. Spend the time to verify your gearing. You may have to take off the diff cover and count the teeth or see if there are marks on the ring gear showing ratio, or you can do the driveshaft rotation test, which is less accurate.

https://midwestjeepwillys.com/speedometer-gears.html

 

If you have an open differential spinning one wheel will only rotate the driveshaft 1/2 of the gear ratio becuse of how the spider gears work. If you rotate the tire once and the driveshaft rotates 1.5 that would be a 3.07 gear ratio which is what the 4.0 5 speed XJs came with. If it rotates a little more than 1.5 times it is a 3.55 gear ratio which is what the 4.0 autos come with.