Speedometer Driven Gear Calculator
by GB Janssen (Black_Magic)

This Gear calculator is mainly for 1963-1967 Corvette 4 speed transmissions (Muncie) but can be used as a guide for other GM 4 speed transmissions.

We are working with the following input variables:

1. Tire diameter – Original C2 Corvette were approx 27.1”. You can usually find your replacement tire diameter on manufactures web site.

2. Drive Gear – Sometimes called “Circle” Gear and is located inside transmission tail housing on the main shaft. Transmission must be removed and some disassembly required to replace. There are three (3) steel drive gears two (2) large and one (1) small.

   • 8 tooth (Large) 1.84” diameter (#3708144) – this is the most common and is for 27 tooth spline. --- For 32 tooth spline use (#3978758)

   • 8 tooth (Small) 1.76” diameter (#3708145)

   • 6 tooth (Large) 1.92” diameter (#3845079) – only works with 20 tooth driven gear (#3860329) which is for 4:56 rear gear.

   • Note 1: To check from outside use a cardboard gage through opening and measure width. 1.84” is .610” wide and 1.76” is .531 wide and both have 8 teeth

   • Note 2: "Large" drive gear is usually found with 3.08 to 3.70 axles and "Small" drive gear is usually found with 3.70 to 4.11 axles

3. Driven Gear – Sometimes called “Pencil” Gear and is also in the tail housing but is removed from outside transmission.

4. Rear End Gear Ratio – Most common are 3:08, 3:36, 3:55, 3:70, 4:11 & 4:56

Adding teeth on the DRIVEN gear will reduce the speedometer reading (cable runs slower). A one-tooth change results in about a 5% change in the reading. Likewise Subtracting teeth on the DRIVEN gear will increase speedometer readings by 5%.

Using LARGE driven gear with LARGE drive gear will result in driven gear being chewed up.

Using Small driven gear with SMALL drive gear will result in either gears not meshing or erratic speedometer readings.