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Speedometer Driven Gear Calculator
by GB Janssen (Black_Magic)

This program calculates number of teeth required for the DRIVEN Gear

in 63-67 Corvette Manual Transmissions & most likely other years
(See Information at bottom of page)

Double click on "field" then fill in your values. Then click when finished.

  Drive Gear Size

            Rear Gear Ratio

            Tire Diameter (Stock Size = ~27.1)


            Tooth Driven Gear Needed

 GM Part # Needed

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.

NOTE: If you have SMALL drive gear (1.76) and need to use SMALL gears or LARGE drive gear (1.84) and need to use LARGE gears you either need to pull transmission and replace drive gear or use a ratio adaptor box in-between the transmission and speedometer cable.

Drive Gear                  Driven Gear                           Ratio Adaptor

© 2007 gbjanssen