Speedometer gear chart

[ThatGrimmJeeper]

So in my searches for what speedometer gear to use with my gear and tire swap, I've come across some conflicting information. I decided to figure out the right way of calculating what you need.

I threw in most of the common tire sizes one may use on an XJ. And I put in all of the usable Dana 44 ratios to go with the tires. I color coded the numbers to match the gears I found online.

Most importantly, I added a correction factor to account for the fact that pretty much every tire is a little smaller than the advertised size. So the gear on this chart should be the right one for your size tire.

This chart works with the NP207/208/228/229/231/242 that came stock in the XJ/MJ, as well as the YJ, TJ, LJ, SJ (80's and newer), ZJ, and WJ. It also works with the Dana 300 in the CJ's. I do believe that the Atlas transfer case also uses the same gears so it should work there too.

 

[ThatGrimmJeeper]

If you are using different size tires and/or different gear ratios than are listed above, you can use this simple formula:

• Speedometer teeth = 270.29 × Axle Gear Ratio ÷ Advertized Tire O.D.
• Speedometer teeth = 262.19 × Axle Gear Ratio ÷ Actual Measured O.D.

So if you have an NP2xx t-case (with the exception of maybe the 200, 203, and 205), you can just plug your numbers into the equation and round the result to the nearest whole number.

If you have another transfer case that uses a different number of teeth on the output shaft (like the Dana 20 that uses 6, 7, 8, or 9 teeth on the shaft depending on where it was used), you can use the following equations:

• Speedometer Gear Teeth = 20.79 × Output Shaft Teeth × Axle Gear Ratio ÷ Advertized Tire O.D.
• Speedometer Gear Teeth = 20.17 × Output Shaft Teeth × Axle Gear Ratio ÷ Actual Measured Tire O.D.

 

[cruiser54]

Great info!!

You mind if I add it to my website's helpful info?

 

[ThatGrimmJeeper]

Sure.

 

[cruiser54]

Thanks.

 

[ID96XJ]

Really useful!
So a green 34 must be pretty common?
Thats what my 96 has and a transfer case I have from a 98 does too.