Speedometer driven gear replacement

[Randy R.]

I have a 67 L79 small block with a 4 speed and positraction I need to replace the worn nylon driven gear. The tank sheet and stamping on the differential case indicate that the gear ratio is 3.55. The speedometer is accurate.

The driven gear that I removed is a 22 tooth green one. The Long Island Corvette catalog says this is for a 3.70 gear ratio and that I should use silver 22 tooth gear and that the green one should not be used with the 3.55 ratio.

The only work done on the rear end was to correct some chattering positraction clutches many years ago.

Which one should I order, Green or Silver?

Thank you,

Randy

[Keith B.]

Re: Speedometer driven gear replacement

did you verify what gear you really have?

[Richard G.]

Re: Speedometer driven gear replacement

I would need the tire size to get the exact gear speedometer ratio. Will also need the tooth count of the drive gear. The Tire Manufacture and size would allow the calculation s to be even closer because then one can look up the published tire revolutions per mile. This is a different value than it would expected if you calculated the tire circumference.
That said, GM typically went for the gear ratio that would make the speedometer indicate slightly slow not fast.
New plastic gears are the only way to go as the seals typically wear a groove in the nylon stem and then they will leak transmission fluid.

Or as you indicated "The speedometer is accurate" just replace it in kind and be done with it.
Rick

[Randy R.]

Re: Speedometer driven gear replacement

Why are there two choices for a 22 tooth gear? They must differ in some way besides color.

[Larry M.]

Re: Speedometer driven gear replacement

Probably size (diameter).

There were two different DRIVE gears used for the C2 Muncie transmissions. One was for wide ratio trans (below 3.70 ratio) and one was for close ratio trans (3.70 ratio and higher). I believe these gears also had a different "tooth" count or gear count, and were slightly different size. They also required specific DRIVEN gears that correctly matched up to the DRIVE gear size.

I assume you have the wide ratio trans. If so, use the DRIVEN gear that corresponds to the wide ratio trans and 3.55 rear gear. Then check the speedometer and make a change if required for accuracy.

Larry

[Edward D.]

Re: Speedometer driven gear replacement

Hell, these are only like $10 each and it takes only a minute to change out. Get both and see which one works best. Use a portable GPS for vehicle speed comparison.

[Jeff B.]

Re: Speedometer driven gear replacement

My 67 has the M21 trans, verified by the transmission tag part number. Rear end is 3.55 ratio. I replaced my driven gear some years ago with the same one that came out of it; a green 22 tooth.

[Jim D.]

Re: Speedometer driven gear replacement

Cars with 3:70 rear gears used a green 22 tooth gear, 3:55 used a red 21 tooth gear and 3:36 used a blue 20 tooth gear.

[Randy R.]

Re: Speedometer driven gear replacement

The M 20 and M 21 transmissions use different drive gears which in turn use different driven gears? Why did a green 22 tooth driven gear listed for use in a close ratio transmission work in my wide ratio transmission?

[Richard G.]

Re: Speedometer driven gear replacement

The transmission output speed is the only one that matters. Another way to think about this is the the output shaft always varies with vehicle speed and it doesn't matter what gear the transmission is in. The gear could even be in the differential but it would cost more for the longer cable. The other ratios only vary the speed between the motor and transmission output shaft and are not related to vehicle speed.
This way its even accurate in reverse gear and even in neural if you are coasting.
Hope this make sense.
Rick

[Joe L.]

Re: Speedometer driven gear replacement

I have a 67 L79 small block with a 4 speed and positraction I need to replace the worn nylon driven gear. The tank sheet and stamping on the differential case indicate that the gear ratio is 3.55. The speedometer is accurate.

The driven gear that I removed is a 22 tooth green one. The Long Island Corvette catalog says this is for a 3.70 gear ratio and that I should use silver 22 tooth gear and that the green one should not be used with the 3.55 ratio.

The only work done on the rear end was to correct some chattering positraction clutches many years ago.

Which one should I order, Green or Silver?

Thank you,

Randy

Randy------


They are incorrect. If you removed a green 22 tooth gear you CANNOT use the silver 22 tooth gear. It will not work, at all. DO NOT ATTEMPT TO INSTALL A SILVER 22 TOOTH GEAR IF YOU REMOVED A PROPERLY FUNCTIONING 22 TOOTH GREEN GEAR.

The 22 tooth silver gear did not even exist in 1967. It came along for the 1970 model year when some Muncie transmissions (mostly those used for 1970 Chevelle SS 454 models) changed to the heavier duty 32 spline main shaft. The 32 spline mainshaft supported only a 1.85" OD speedometer DRIVE gear. This DRIVE gear could only be used with the 0.80" OD series DRIVEN gears (18-21 tooth). In order to allow for the accommodation of rear drive ratios of up to 3.70:1 without the use of a speedometer adapter the 22 tooth silver driven gear was released. This gear would work with the 1.85" OD drive gear. All Corvettes went to the 32 spline mainshaft Muncie transmissions for the 1971 model year. Depending upon installed rear drive ratios up to 3.70:1, a DRIVEN gear in the now 18-22 tooth series is used. For those few cars with rear ratios numerically higher than 3.70:1, a speedometer adapter had to be used.

GM says the 21 tooth red gear is the correct one for your 3.55:1 rear gear. However, if your speedometer is currently accurate with the 22 tooth green gear, then that's the gear you ought to use in replacement. Your tire size might make the 22 tooth green gear better for your speedometer accuracy.

[Joe L.]

Re: Speedometer driven gear replacement

The M 20 and M 21 transmissions use different drive gears which in turn use different driven gears? Why did a green 22 tooth driven gear listed for use in a close ratio transmission work in my wide ratio transmission?

Randy------

Whether the transmission is an M-20, M-21, or M-22 has no bearing, at all, on the speedometer drive or driven gears. The only factor that has a bearing on these gears is the installed rear end ratio.

[Joe L.]

Re: Speedometer driven gear replacement

Randy-------


I had another thought on this. You will note that in my response, above, I said: DO NOT ATTEMPT TO INSTALL A 22 TOOTH SILVER GEAR IF YOU REMOVED A PROPERLY FUNCTIONING 22 TOOTH GREEN GEAR. I've added the emphasis on a very important part of that statement. What is the condition of the green gear that you removed? You mention that it is "worn". Is it "chewed up" in any way or has its OD been reduced? Can you post a photo of it? Normally, these gears do not wear in a noticeable sort of way.

Here's another possibility I thought of: your car with a 3.55:1 rear ratio should have originally had installed a red 21 tooth driven gear. It's possible that at some point due to speedometer inaccuracy, possibly due to non-original OD tires, someone decided that they needed to install a 22 tooth driven gear. So, they installed a GREEN 22 tooth gear. That's when the mistake was made. A GREEN 22 tooth gear would not be compatible with the installed DRIVE gear which can be inferred if the car was originally equipped with a red 21 tooth driven gear. Although no 22 tooth silver driven gear was ever available in 1967, it would be the correct gear to use now IF YOUR CAR WAS ORIGINALLY EQUIPPED WITH A 21 TOOTH RED GEAR AND NOW REQUIRES A 22 TOOTH DRIVEN GEAR FOR SPEEDOMETER ACCURACY. In that case, you got the correct information from Long Island Corvette and my apologies for contradicting them.

The thing that influenced my earlier response is the fact that, apparently, the 22 tooth green gear had been functioning OK, presumably for quite some period of time. That would really surprise me. That's because if a 22 tooth green gear were installed in a transmission which originally had a 21 tooth red gear installed, I would expect the green gear to be destroyed and rendered inoperable rather quickly.

Once again, if you could post a good close-up photo of the green gear your removed it would be very helpful in getting to the bottom of this.

[Wayne M.]

Re: Speedometer driven gear replacement

Per TSB in Spring 1967, with 7.75 rubber. Factory installation would have been the green 22 tooth, irrespective of whether 3.35 or 3.70 final ratio. Combo 5C on the attached tables. I know this sounds illogical, and it implies that the speedo will read high in one case and low in another. Probably because the final ratio gaps are roughly 5% (3.70 to 3.55, and 3.55 to 3.36), whereas the other final ratios are around 10%.



Sorry about pic orientation -- it was fine on the original file.

[Richard G.]

Re: Speedometer driven gear replacement

I did the math.
The speedometer cable should turn 1000rpm at 60mph. This is the speed the speedometers are calibrated to.
With the 3:55 gears it turns 1000.45 rpm
With the 3:70 gears it turns 1042.7 rpm

In this case the speedometer should be dead on at with the 22T gear and the 3:55's
And slightly fast with the 3:70's.

One of the very few instances, that I have calculated, where the speedometer is calibrated to read higher than actual speed.
Rick