Classic Mustang speedometers are spun by a steel cable that runs from the transmission to the speedometer. The rate at which the speedometer spins is determined by a drive gear that is either part of the transmission output shaft (in the case of automatic transmissions) or physically attached to the transmission output shaft (in the case of manual transmissions) and a driven gear that's attached to the end of the speedometer cable. Different combinations of drive- and driven-gear teeth are used to produce reasonably accurate speedometer readings given the different tire, gear, and transmission combinations. Driven gears are also available in two helix (spiral direction) configurations. Left helix gears as used in original 4-speed manual transmissions are made of whitish nylon with a color-coded tip. Right helix gears as used in original automatic and 3-speed manual transmissions are made of colored nylon. Removable drive gears are also made of colored nylon. Other transmissions might not follow these patterns.
Removable drive gears are secured to the transmission output shaft with a snap ring and a small ball bearing that rides in a groove on the inside of the gear. The pink 6-tooth, C5ZZ-17285-A drive gear and the black 7-tooth, C4DZ-17285-A drive gear can be seen in the images below:
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| C5ZZ-17285-A Drive Gear, Top View | C5ZZ-17285-A Drive Gear, Side View | C4DZ-17285-A Drive Gear, Side View (Image courtesy of Scott Hollenbeck) |
The black drive gear was used for some high axle ratio applications, and, interestingly enough, for several low axle ratio applications. As the transmission output shaft rotates, these drive gears turn the driven gear that's attached to the speedometer cable. Here's an example of the 18-tooth, C2DZ-17271-F driven gear that's commonly used with 3.91:1 rear end gears. Note the green color on the tip.
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| C2DZ-17271-F 18-tooth Driven Gear, Green, Top View | C2DZ-17271-F 18-tooth Driven Gear, Green, Side View |
As you might expect, different gear combinations are needed to keep the speedometer accurate with other axle ratios. Here's an example of the 20-tooth, C2DZ-17271-H driven gear that's commonly used with 4.30:1 rear end gears. Note the blue color on the tip.
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| C2DZ-17271-H 20-tooth Driven Gear | C2DZ-17271-H 20-tooth Driven Gear, Blue, Side View |
The table below describes the different combinations of speedometer drive and driven gears used on 428 Cobra Jet Mustangs equipped with 3.91:1 and 4.30:1 rear end gear ratios using information found in Master Parts Catalogs (MPCs) from the 1968 and 1969 model years, as well as the 1975 "Final Edition" catalog. Note that some of the applications require use of an adapter.
| Year, Ratio, Transmission | Tire Size | Drive Gear | Drive Gear Teeth | Drive Gear Color | Driven Gear | Driven Gear Teeth | Driven Gear Color |
|---|---|---|---|---|---|---|---|
| 1968, 3.91, Automatic | FR70 x 14 | $ | 8 | C7AZ-17271-B C7SZ-17271-A |
17 17 |
Natural Green |
|
| 1968, 3.91, Manual | FR70 x 14 | C5ZZ-17285-A | 6 | Pink | C2DZ-17271-F C2DZ-17271-H |
181 202 |
Green Blue |
| 1968, 4.30, Automatic | FR70 x 14 | $ | 8 | C6VY-17271-A C7SZ-17271-B |
18 18 |
Gray Gray |
|
| 1968, 4.30, Manual | FR70 x 14 | C5ZZ-17285-A | 6 | Pink | C2DZ-17271-H | 20 | Blue |
| 1969, 3.91, Automatic | 7.35 x 14 E70 x 14 F70 x 14 FR70 x 14 E78 x 14 |
$ | 8# | C7AZ-17271-B C7SZ-17271-A |
17 17 |
Natural Green |
|
| 1969, 3.91, Manual | 7.35 x 14 E70 x 14 F70 x 14 FR70 x 14 E78 x 14 |
C4DZ-17285-A | 7% | Black | C2DZ-17271-J | 16 | Orange |
| 1969, 3.91, Manual | 7.35 x 14 7.75 x 14 E70 x 14 F70 x 14 FR70 x 14 |
C5ZZ-17285-A | 6 | Pink | C2DZ-17271-F | 18 | Green |
| 1969, 4.30, Automatic | 7.35 x 14 E70 x 14 E78 x 14 |
$ | 8# | C6VY-17271-B | 19 | Tan | |
| 1969, 4.30, Automatic | F70 x 14 FR70 x 14 |
$ | 8# | C6VY-17271-A | 18 | Gray | |
| 1969, 4.30, Manual | 7.35 x 14 E70 x 14 E78 x 14 |
C4DZ-17285-A | 7% | Black | C2DZ-17271-F | 18 | Green |
| 1969, 4.30, Manual | F70 x 14 FR70 x 14 |
C4DZ-17285-A | 7% | Black | C2DZ-17271-G | 17 | Purple |
| 1969, 4.30, Manual | 7.35 x 14 7.75 x 14 E70 x 14 F70 x 14 FR70 x 14 |
C5ZZ-17285-A | 6 | Pink | C2DZ-17271-H | 20 | Blue |
| 1970, 3.91, Automatic | 7.35 x 14 7.75 x 14 E70 x 14 F70 x 14 E78 x 14 F60 x 15 |
$ | 8# | C7SZ-17271-A | 19 | Green | |
| 1970, 3.91, Manual | 7.35 x 14 7.75 x 14 E70 x 14 F70 x 14 E78 x 14 F78 x 14 G70 x 14 G78 x 14 F60 x 15 |
C4DZ-17285-A | 7% | Black | C2DZ-17271-J | 16 | Orange |
| 1970, 4.30, Automatic | 7.35 x 14 E70 x 14 E78 x 14 |
$ | 8# | C7VY-17271-A | 19 | Tan | |
| 1970, 4.30, Automatic | 7.75 x 14 F70 x 14 F78 x 14 G70 x 14 F60 x 15 |
$ | 8# | C7SZ-17271-B | 18 | Gray | |
| 1970, 4.30, Manual | 7.35 x 14 7.75 x 14 E70 x 14 E78 x 14 |
C4DZ-17285-A | 7% | Black | C2DZ-17271-F | 18 | Green |
| 1970, 4.30, Manual | F70 x 14 F78 x 14 G70 x 14 F60 x 15 |
C4DZ-17285-A | 7% | Black | C2DZ-17271-G | 17 | Purple |
Notes: All listed applications exclude rental, police, and taxi vehicles.
1,2: Both of these driven gears are listed in a Master Parts Catalog (MPC) dated August, 1968. The 20-tooth driven gear can't work with both the 3.91 and 4.30 rear gears, so the 20-tooth gear may be incorrect.
$: Part of transmission output shaft
#: 8-tooth drive gear in combination with an externally mounted 0.6944 geared ratio adapter
%: 7-tooth drive gear in combination with an externally mounted 0.750 geared ratio adapter
Note that modern tires are sized differently, and it can be difficult to determine an optimal speedometer gear combination using the chart above if you're using modern tires. We have an interactive speedometer gear calculator that produces recommendations for the best combinations of drive and driven gears for modern tire sizes.
Images are provided courtesy of Chris Teeling unless otherwise noted.