Help with rear differential

I am trying to figure out why my 69 CJ5 is reading the wrong speed (faster than stated speed) It came with 31 inch tires (not sure what OEM was)

The rear differential for 69 was 3.73 or 4.88 I understand. I took a picture of my rear differential and I think it says 4.27. Can anyone verify if I am correct. What model would this be? Is 4.27 the gear ratio? Since I am keeping 31 inch tires should I buy a different speedometer drive gear to match?
Thanks

 

that is 4.27 markings.

 

Wierd- I think 4.27 was an option on the F head engine. So is it more likely that the previous owner changed the rears to the 4.27 or converted the engine to the 225 that is in the jeep? I don't know much about it's history. Either way- I will need to change the speedomemter drive gear to make it read correctly, right?

 

47 11 -> 47/11 = 4.2727.

Depends on whether it has the proper 4.27:1 speedo gear or not. If the chassis started out with 4.27s then it has the right gear already.

A 225 conversion is more likely than an axle swap, though both are possible.

 

Either way I need to change the speedo gear because it is reading faster than I am going right? Do you know how many drive gear teeth it has so I can order the correct number of driven gear teeth?

 

Nope. If the chassis came with 4.27s and the transfer case has not been changed, then the speedo gear does not change.

I think that Jim (jpflat2a) posted the tooth count information in the thread that's linked in your other thread.

Courtesy Patrick http://www.earlycj5.com/forums/showthread.php?t=12125&highlight=speedo+gear+part+numbers

 

Here is a link to some good information. Click on the axle you have and will give the tooth count on the ring and pinion

http://www.precisiongear.com/danaring.htm

Steve

 

Simple solution. Use a GPS which displays mph and compare to the reading on your speedometer. Use marking tape to mark the important numbers on your speedometer. 30, 40, 50 and maybe 60.

My 68 has 6.00 - 16 tires on which are 28" diameter. Yours are 31.

3" difference, so about 10% off (3/28). The circumference of a circle is proportional to the tire diameter. (C=PI x diameter)

So if your speedometer is saying 30, you would actually be going 33 MPH.