Speedometer Gears

[Lawrence Berte]

Anyone know which drive and driven speedometer gears are needed to change 2- C3s, both with M21 wide ratio transmissions. One car had 3.08, the other 3.36. They are both now 3.55.

[Joe L.]

Re: Speedometer Gears

Lawrence-----

Providing that the transmissions were ORIGINALLY set-up for use with the 3.08 and 3.36 rear gears, you should have a usable DRIVE gear in both transmissions. Changing the DRIVE gear can be difficult, so that part is good.

You should pull a 19 tooth natural-colored DRIVEN gear out of the car with 3.08s and a blue 20 tooth gear out of the car with 3.36s. These will confirm that you have the proper DRIVE gear installed in both.

Change the DRIVEN gear in both cars to a 22 tooth SILVER gear of GM #3987922 and you should have acceptable speedo calibration for 3.55s. Of course, this assumes original tire diameter.

[Steve L.]

Re: Speedometer Gears

Any idea why my plastic brown gear would wear out on one side at about every 35k miles. About 5 teeth would be completely mushed down.

Steve L
73 coupe since new

[Joe L.]

Re: Speedometer Gears

Steve----

My guess: the speedometer DRIVE gear is mis-located on the main shaft. For all 1957-63 T-10s, 63-67 Muncies, 71-74 Muncies, and 74+ T-10s, the speedometer DRIVE gear is a metal gear which is pressed on the mainshaft. There is absolutely nothing on the mainshaft which indexes it or provides for its exact location. There is a significant range of area possible for final placement of the gear on the journal on the mainshaft. However, if the gear is not located almost exactly correctly, speedo gear wear, or worse, can result. This is a common speedo gear problem on these transmissions. Why a more positive means of indexing the gear was not used is beyond me.

1968-70 Muncie owners don't need to worry about this problem, though. For those model year transmissions ONLY, the speedometer DRIVE gear was a nylon gear which indexed to the shaft via a specially machined not on the shaft and a special clip which retained the gear. So, for these transmissions, perfect alignment of the DRIVE gear is assured. But, those are the only Corvette (and other GM) 4 speeds that are "so-blessed".

[Lawrence Berte]

Re: Speedometer Gears

Thank You Joe, the driven gears you recomended are now on order. Lawrence.

[Joe L.]

Re: Speedometer Gears

Lawrence----

Be advised that the gears that I recommended will likely give the CLOSEST to true speedometer reading. However, the readings will rarely be exact. Accuracy within 2% is about the best one can ever expect from a stock, mechanical speedometer system. Each tooth of DRIVEN gear count will change the reading by about 5%. So, one often ends up "in between" somewhere.

"Certified" speedometers, such as those used in police cars, won't do too much better than 2%, either. But, they're usually certified to be at 2%, or less error. In the case of mechanical speedometers, this is usually done by installing a speeedometer adapter which can provide a very precise "correction factor" to the speedo DRIVEN gear's output (and, consequently, speedometer reading).

[Steve L.]

Re: Speedometer Gears

Thanks Joe,
This is what I suspected because the speedo cable turned very easily by hand.

I purchase 3 of them (73 - 4 speed) at one time since it seemed like a regular maintenance item and it took forever to get them from the dealer in those days.

Is there a fix or workaround for this problem. I would kind of dought it.

Steve L
73 coupe since new

[Joe L.]

Re: Speedometer Gears

Steve----

The only fix is to reposition the DRIVE gear on the mainshaft. That involves removal of the transmission and disassembly. So, it's best left until trans rebuild time as long as you can get 35,000 miles out of a DRIVEN gear as it now stands.