Speedo drive gear - M-20 - 3.70 rear

[Richard D.]

Looking to find out what color gear I need and number of teeth on my 71+ (73 I think) M-20 running 3.70 gears in the back.

Thanks for any help,

Rich

[Joe L.]

Re: Speedo drive gear - M-20 - 3.70 rear

Rich-----

Assuming that you are using tires with an OD of close to 27-1/2", you should be able to get by with a 22 tooth, SILVER speedometer DRIVEN gear of GM #3987922.

If the above doesn't give acceptable speedo calibration, then you will need to use an 18 tooth, BROWN gear of GM #3987918 PLUS a speedometer adapter of GM #1565211 which has a 0.8824 ratio.

For 71-74 Muncies, rear gear ratios of 3.70:1 sometimes require an adapter. Ratios numerically lower than 3.70:1 ALWAYS require an adapter.

[Richard D.]

Re: Speedo drive gear - M-20 - 3.70 rear

Thanks Joe, appreciate the help.

Regards,

Rich

[Clark Chapman]

Re: Speedo drive gear - M-20 - 3.70 rear

Once you pick a gear and if you find your calibration is off. Keep in mind you can opt to get a different tooth count gear. For every added tooth you drop roughly 3 mph on the odemeter. Chevrolet has these gears from 18 teeth to 23 teeth. If you find you still can't calibrate your OD then you may need to get the adapter mentioned in the ealier reply. Good luck.
Clark

[Joe L.]

Re: Speedo drive gear - M-20 - 3.70 rear

Clark-----

Speedometer driven gears are available up to 25 tooth count. However, all of the 23-25 tooth gears, as well as the 22 tooth green gear are incompatible with the DRIVE gear used for all 71-74 Muncies. The 22 tooth SILVER gear is the maximum tooth count gear that can be used with the 71-74 Muncie DRIVE gear. Since ratios lower than about 3.70:1 require DRIVEN gear tooth counts greater than 22 teeth, an adapter must be used to achieve correct speedometer calibration. That's also how GM did it with 71-74 Muncies.