speedometer drive gear

[Kermit Wilke]

RE: 1978 anniversary L82 4-speed. Is it necessary to remove the transmission to replace the speedometer drive gear?

[Joe L.]

Re: speedometer drive gear

Kermit------

If you are, indeed, talking about the speedometer DRIVE gear, the answer is yes. The speedometer DRIVE gear is mounted on the mainshaft. For 1978, it is a press fit steel gear. You must remove the transmission from the car and, at least, partially disassemble it to service the DRIVE gear.

If you are talking about the speedometer DRIVEN gear (which some folks call the drive gear since they reason that it "drives" the speedometer), then the answer is no. The DRIVEN gear, which is a nylon gear with a stem, can easily be removed from the car by simply removing the small bolt and retainer which secure the speedometer adapter to the transmission. Pull out the adapter, pull out the gear, insert a new gear, and you're done. One word of caution, though, is that you cannot use ANY tooth count driven gear with your installed DRIVE gear. For your 78 application and factory-installed DRIVE gear, you can only use 18 to 21 tooth gears or a SILVER-colored 22 tooth gear.

[Kermit Wilke]

Re: speedometer drive gear

Thanks Joe. That is about what I expected and I could not see any way to remove the tail shaft while the transmission is in the car. The drive gear turns on the shaft. Apparently what happened, was that the cable housing was too close to the exhaust and melted to the exhaust pipe. Instead of twisting off the cable or striping the driven gear, the drive gear let go.

[Mike M.]

Re: speedometer drive gear

kermit & joe. I'm not a c-3 expert, but i think by then, the drive gear wasnot a press fit on the tailshaft, but rather retained by a spring steel clip. also seem to recall the drive gear being nylon. could be wrong. in any event , think you'll have to pull the tranny to fix the problem. mike

[Kermit Wilke]

Re: speedometer drive gear

I was guessing nylon, but can't get a good look. I can turn the gear with my finger. It will turn the cable up to about 10-15 MPH.

[Joe L.]

Re: speedometer drive gear

Mike and Kermit-----

As far as manual transmissions go, the nylon speedometer DRIVE gear was used only for 1968-70. For these years, there were 2 different nylon drive gears. The "small" diameter was medium blue in color; the "large" diameter was light green in color. Both of these gears were retained and located by a metal retainer which locked into a special machining found only on the 68-70 Muncie main shafts (and 64-67 SERVICE mainshafts after about 1969).

Prior to 67 and from 71-81, the speedometer DRIVE gears for manual transmissions were steel gears which were an interference fit on the main shaft. For 57-67 there were 2 different steel gears which in size and diameter were generally configured just like the 68-70 nylon gears. For 71-81, the gear had a larger ID to fit the larger 71+ mainshaft OD. However, just one gear OD was available. This was the same as the "light green" 68-70 gear and the corresponding steel 57-67 gear. Thess gears worked with rear ratios of approximately 3.55:1 and numerically lower. For higher numerical gear ratios, which were not used very much for 71-81, a speedometer adapter was used.

[Wayne M.]

What about 6-tooth 4.56 + drive gear (to 1969)

And how was this handled in the '69 - '70 nylon gear era ?

[Joe L.]

Re: What about 6-tooth 4.56 + drive gear (to 1969)

Wayne-----

Yes, there was a third steel gear used for pre-68 applications with 4.56-up rear axle ratios. It was a 6 tooth gear of GM #3845079.

For 68-70, speedometer adapters were used for ratios of 4.56:1 and numerically higher. Speedometer adapters are expensive. But, considering the small number of cars requiring them and the fact that they eliminated the need for more transmission part numbers on the assembly line (differing by only the installed speedometer DRIVE gear), I'm sure that GM considered that there was an overall economy.