I’m replacing the speedometer gear in my 65 442 because I have converted the M-21 to a M-20 because it is more compatible with the 3.23 rear end. I replaced the gear to a 19 tooth gear that the CSM chart suggests. The original gear has 22 teeth and there is no listing on the chart for any such gear. Is anyone familiar with this particular gear?

 

https://www.thegearbox.org/catalog/i...82/3697857.htm

 

Thanks for that info, how do I know what drive gear there is inside the trans?

 

https://www.cartechbooks.com/techtip...e-speedometer/

Most transmissions use different colored drive gears to indicate how many teeth it has, apparently the Muncie transmissions use 3 different drive gears, 6, 7 and 8 tooth of 2 different diameters, all the same color. The link above shows how to tell the difference. Be sure the driven gear is compatible w/ the diameter of the drive gear.

Having said all that, it seems that if you swap both the drive and driven gears from your old trans, you should be good.

 

That’s very interesting! My dilemma is that my speedo doesn’t work at all. Let me go into detail what I have done. My car originally came with the Jetaway and I put a Muncie 4 sp. that came out of another 65 442. (Original M-21 but converted to a M-20). The rear end is the 3.23 posi. The rear end that was in the parts car was a 3.55. I wasn’t aware of the difference of the drive gear when I rebuilt the trans. So could this be my problem? So if I put the 22 tooth driven gear back in and remove the 19 tooth gear. How much will that change the accuracy of the speedometer? Do you understand what I am saying.

 

If you look at the link I posted it tells you what driven gears need for drive gears.

 

I saw that, but how do I know what drive gear I have? The trans is all put together.

 

I assume the 3.27 listed in the chart is a mis-print and should be 3.23

 

You can try pulling the driven gear out and looking in to see the drive gear, then count the teeth as you rotate the driveshaft. However, it may still be hard to determine the diameter of the drive gear without removing it from the trans. You should be able to infer the diameter of the drive gear from the diameter of the driven gear: A driven gear with diameter of 0.810" is used w/ a 1.84" drive gear while a 0.870" diameter driven gear is used w/ a 1.76" dia drive gear. If your speedometer is not working (and you are sure the speedo and cable are OK), it may be possible that you have a small driven gear AND a small drive gear and they simply are not engaging (NOTE: There is a 22 tooth driven gear available for both size drive gears.......check the color codes in the link svnt442 provided). This could be confirmed by removing the speedo cable and rotating the drive shaft to see if the driven gear spins.

You have not provided sufficient information to be able to determine the proper driven gear you need, we need:
- Rear tire diameter
- Rear gear ratio (3.23?)
- # teeth on the drive gear

The effect between a 19 and 22 tooth driven gear depends on how many teeth the drive gear has.

 

I’ll check the operation of the speedometer and cable and go from there. I’ll continue to test by what you have suggested. I’m trying to avoid pulling the trans to get to the bottom of this. One other thing that I might not have done when I assembled the trans was not have the drive gear in the exact location it was originally put on, although I remember putting it on the same spot it appeared to be on. Can the drive gear be installed backwards? Thanks for you help!

 

What year is your M20 ?
I am in the same boat, working on it.

Most 1969–1970 Muncies came with an eight-tooth plastic drive gear and clip. These do not fit well on the shaft. If your cable or speedometer locks up, the clip breaks, and the gear becomes dislodged; this requires transmission removal and partial disassembly. It is the same diameter as a 1.84-inch steel gear. Always upgrade to a steel drive gear if you are doing a rebuild.

copied from here

https://www.cartechbooks.com/techtip...e-speedometer/

 

The Muncie is a 65. Was there a common size drive gear for the 65’s?

 

You know what DIAMETER gear is in there based on the gear that you pulled out of it.

 

Well that narrows it down, I’ll take a look at the gear that was originally in the trans and get a diameter on it.

 

Likely the 1.76, as the tech article state they starting using the 1.8 in the later because automatics shared the same.
However I am NOT sure.
Mine is a 69 m20.

 

The tooth count of the gear you pulled out (22 tooth) will tell you the diameter of the drive gear. It will be the smallest in diameter and as such the 19 tooth gear won't make contact with it.

EDIT: I just noticed that there are two different diameter 22 tooth driven gears. You must have the larger one for use with a smaller diameter driver dear. The 19 tooth gear will require a drive gear change.

 

I wish I knew that when I rebuilt the trans, I guess ya learn something new every day. For now I’m just gonna put the 22 tooth gear back in. I will eventually do the switch on the drive gear. I can live with the speedo being around 5 mph off if that’s all it will be? Thanks for everyone’s help!

 

It's a percentage off, not a specific MPH. So the faster you go, the more off it will be.