by Paul Cangialosi
I’ve seen many people frustrated because they took the time to do a thorough job rebuilding a used Muncie, but later, after the transmission is installed, they find out that the speedometer is reading incorrectly. When I build a new transmission or do a rebuild, I always ask the customer these four questions:
What is the tire size you plan to use? (Example: 255/60/15)
If you don’t know the tire size, do you know the diameter?
What is the axle ratio?
Do you know the tooth count of the speedometer drive gear in the transmission?
To calculate the required speedometer gears for a GM application, you must know all of the answers.
Formula for Calculating Tire Diameter
Using the example above, 255/60/15 means that the tire has a section width of 255 mm. It has an aspect ratio of 60, which is the ratio of sidewall height to section width. The final number, 15, is the actual wheel diameter.
Here’s the formula for calculating tire diameter:
Tire Diameter = (2 x sidewall height) + wheel diameter
First you have to convert the section width to inches. You, divide it by 25.4. In our example:
Section Width = 255 mm ÷ 25.4 = 10 inches
Next, you find the sidewall height. You multiply the section width by the aspect ratio. In our example:
Sidewall Height = 10 x .60 = 6 inches
Using the formula, our example tire diameter is 27 inches.
(6 x 2) + 15 = 27 inches
Formula for Calculating Speedometer Driven Teeth
Driven-Gear Tooth Count = (speedo drive-gear tooth count x axle ratio x tire revs per mile) ÷ 1,001
The formula to calculate tire revolutions per mile is:
Tire Revs Per Mile = 20,168 ÷ tire diameter (inches)
Using the tire example above:
20,168 ÷ 27 = 747 revs per mile
Before calculating the driven-gear tooth count, the drive-gear tooth count must be determined.
Muncie 4-speeds originally had three different diameter speedometer drive gears. The six-tooth gear (no longer available) had no driven-gear options except for one steel 20T pencil gear.
The parts resource guide lists two major diameter drive gears: 1.76 and 1.84 inches.
Driven pencil gears come in tooth count ranges that coincide with the size of the drive gear.
General Motors discontinued the 1.76-inch drive gears and the corresponding driven gears. Because the 1.84-inch diameter was used in automatics, the cable-threaded fittings and driven pencil gears have been reproduced. AGE makes additional 7- and 9-tooth gears to work with these standardized pencil gears. It is now possible to calibrate your speedometer for a wider combination of rear axle and tire sizes.
So, if you have an eight-tooth drive gear and use the above equation with an axle ratio of 3.73:1 you get 22:
(8 x 3.73 x 747) ÷ 1,001 = 22
If you use a large-diameter eight-tooth gear, you may want to switch to a seven-tooth gear. In this case you get 19 or 20 (depending on how you “round” the answer):
(7 x 3.73 x 747) ÷ 1001 = 19.48
Because a seven-tooth 1.84-inch-diameter gear is now available, using one puts you more in the middle range of driven pencil gears; it also gives you more options.
These are the drive gears. From left to right: 1.84-inch 32-spline output shaft, 1.84-inch 27-spline output shaft, and 1.76-inch 27-spline output shaft.
The drive-gear’s tooth count is determined by laying the gear on its side and marking where each spiral-worm tooth comes to an end. This gear has eight teeth and is 1.76 inches in diameter.
Notice that the diameter of the blue gear is smaller than that of the yellow gear. Small-diameter pencils work with 1.84-inch drive gears; the range is 17 to 22 teeth. Large-diameter pencils work with 1.76-inch drive gears; the range is 22 to 25 teeth.
Most 1969–1970 Muncies came with an eight-tooth plastic drive gear and clip. These do not fit well on the shaft. If your cable or speedometer locks up, the clip breaks, and the gear becomes dislodged; this requires transmission removal and partial disassembly. It is the same diameter as a 1.84-inch steel gear. Always upgrade to a steel drive gear if you are doing a rebuild.