Calculate the speedometer driven gear tooth count from drive gear teeth, axle ratio and tire size, or check speedometer error after a gear or tire change.
- All Automotive Calculators
- Odometer & Mileage Correction Calculator
- Staggered Tire Calculator
- Driveshaft U-Joint Operating Angle Calculator
Speedometer Driven Gear Formula
A cable-driven speedometer reads correctly only when the speedometer cable turns a set number of times per mile. When you change tire size or the axle ratio, that turn rate changes, so you swap the driven gear to bring it back. To find the driven gear teeth you need, the calculator uses:
If measured tire revolutions per mile are unavailable, a geometric estimate uses tire diameter in inches:
To check the error of a set of gears you already have installed, it compares the ideal driven gear count with the driven gear that is actually fitted:
Variables:
- Nd is the ideal driven gear tooth count before whole-tooth rounding
- Dr is the number of teeth on the drive gear at the transmission output shaft
- A is the axle (rear end) ratio, such as 3.73
- R is the tire revolutions per mile
- D is the overall tire diameter in inches
- C is the instrument calibration in cable revolutions per mile. The default is 1000; 1001 is an alternative formula convention, not a Ford-only value. Confirm your instrument specification.
- E is the speedometer error as a percent, where a positive value reads high (fast) and a negative value reads low (slow)
- Ndinstalled is the driven gear tooth count currently in the transmission
In “Driven gear tooth count” mode, enter the drive teeth, axle ratio and tire diameter or measured revolutions per mile. The result is the nearest whole count to the unrounded ideal, with a minimum of one tooth. It does not guarantee availability, fit or zero error; the details compare both neighboring counts and their modeled errors. In “Speedometer error” mode, also enter the installed driven teeth to check the reading. Optional settings select cable calibration and reference speed, which defaults to 60 mph. Blank reference speed uses 60; zero gives zero speed examples without changing percentage error.
GM Speedometer Gear Tooth Ranges and Sensitivity
These are examples listed in the TCI gear reference, not universal fit ranges. Confirm the exact application, drive/driven pairing and housing. The final column gives approximate sensitivity; smaller gears change the reading more per tooth.
| Transmission family | Driven gear teeth | Example colors | Change per tooth |
|---|---|---|---|
| TH350 small-housing examples | 18 to 22 listed; verify fit | Brown 18, Red 21 | about 5% |
| TH350 BOP / TH400 / 700R4 large-gear applications | 34 to 45; housings 34 to 39 or 40 to 45 | White 36, Black 40, Green 42 | about 2.5% |
The drive gear sits on the output shaft. The following listed examples do not identify your installed gear; count its teeth and confirm the specific transmission and parts combination.
| Transmission | Typical drive gear teeth |
|---|---|
| TH350 examples | 7 to 10 or 17 to 20 |
| TH400 examples | 7 to 8 or 15 to 21 |
| 700R4 examples | 15, 17 to 20 |
| Powerglide | Count installed gear; confirm application |
| Muncie / Saginaw manual | varies, count to confirm |
With all other inputs fixed, adding driven teeth lowers the indicated speed and removing teeth raises it. Verify actual readings after a change; a rounded tooth count does not guarantee an exact reading or establish physical compatibility.
Example Problems
Example 1: Find the driven gear for a tire change.
For an 8-tooth drive gear, a 3.73 axle ratio, a geometric 28-inch tire diameter and 1000 cable rev/mile calibration, R = 63360/(π × 28) ≈ 720.289800 rev/mile. The ideal driven count is approximately 21.493448, which rounds to 21 teeth before display rounding. At 21 teeth the modeled error is +2.349751%, so actual 60 mph indicates about 61.409850 mph. The adjacent 22-tooth count gives −2.302511% error, slightly smaller in absolute percentage terms. Neither count is a fit guarantee.
Example 2: Check a set of gears you already have.
With a 17-tooth drive gear, a 3.08 axle ratio, a geometric 29-inch tire diameter and 1000 cable rev/mile calibration, R ≈ 695.452220 rev/mile and the ideal driven count is approximately 36.413878. An installed 40-tooth gear gives −8.965304% modeled error: actual 60 mph indicates about 54.620817 mph, while indicated 60 mph corresponds to about 65.908937 mph actual speed. The nearest whole count, 36 teeth, still gives +1.149662% error; 37 teeth gives −1.584113%. Whole-tooth rounding does not eliminate error.
Frequently Asked Questions
Do I add or remove teeth if my speedometer reads too fast?
For the same drive gear, axle ratio, tires and instrument calibration, adding driven teeth reduces the reading; removing teeth raises it. Starting from n driven teeth, adding one changes the reading by −100/(n + 1)% relative to the previous reading. Removing one changes it by +100/(n − 1)% when n is greater than one. These are changes in the reading, not percentage-point corrections to every existing error.
Where are the drive and driven gears located?
In many cable-driven systems, the drive gear is on the transmission output shaft and the driven gear is in a removable housing or sleeve connected to the cable. Attachment and service procedures vary; identify the exact transmission and consult its service instructions before changing parts.
Does this calculator work for an electronic speedometer?
This model evaluates the gear ratio for a cable-driven speedometer. Electronic instruments use pulses-per-mile calibration, and some sensors are themselves gear-driven. Follow the instrument and transmission manufacturer’s calibration procedure; a calculated gear count alone does not establish the correct electronic pulses-per-mile setting.
