Speedometer Drven Gear

[Richard P.]

I'm having a problem with my Muncie 4-speed chewing up the plastic driven gear. The last one only lasted about 100 miles. Is there an adjustment for the drive gear in the tailshaft ? The driven gear is worn in the center of the teeth.
Thanks for any input.

[Roy B.]

Re: Speedometer Drven Gear

The " drive gear has moved" or loose,or the driven gear housing holding the plastic gear is bad???

[Wayne M.]

Sounds like wrong Drive / Driven (plastic) combo

Could be what Roy has suggested, but symptoms indicate use of a small diameter 8-tooth steel drive gear (on the trans tailshaft) intended for final drive ratios 3.70 and 4.11) in combination with the small diameter plastic driven gear (ratios 3.08 thru 3.55). The two gears barely mesh at the center of the plastic gear, and result in wear and skipping after a short time.

The large dia. driven gear (1.850" x 0.610" thk.) #3780144 takes the small dia. plastic gears, and the small dia. drive (1.765" x 0.531" thk) #3780145 takes the larger dia. plastic gears. I don't think a large drive / large driven will even fit.

Did you change the plastic gear recently (to match a differential final drive ratio change) ? Or, did you install a different transmission without verifying what speedo drive gear it had ?

4.56 final drive ratios require yet another 6-tooth drive gear and different driven gears. This is why (example) when the M-22 was introduced in spring of 1965, Chevrolet required 3 different transmissions #'s [on the side cover tags] to cover the 3.08 to 4.56 span.

P.S. there's a relatively recent 22-tooth silver/gray small diameter plastic gear for 3.70 ratios # 3987922 that can be used with the large dia. drive gear; previously, only the green 22-tooth large dia.plasitc gear with the small dia. drive gear would do for this ratio.

[Richard P.]

Re: Sounds like wrong Drive / Driven (plastic) com

Thanks for the insight. I did change to a 20 tooth (3.55 ratio) driven gear. I hadn't checked the gear I removed, but it sure sounds like a small/small combination. Thanks again for your help.

[John H.]

Re: Sounds like wrong Drive / Driven (plastic) com

Rick -

The 20-tooth blue driven gear is in the "small" family (requires the "large" 1.85" drive gear on the mainshaft), and is for a 3.36 axle.