Calculate the driven gear teeth to recalibrate your speedometer from drive gear, axle ratio, and tire size, and check your speedometer error after a change.
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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:
Nd = (Dr * A * R) / C
where the tire revolutions per mile come from the tire diameter:
R = 20168 / D
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:
E = (Nd_ideal / Nd_installed - 1) * 100
Variables:
- Nd is the number of teeth on the driven gear you need
- 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 speedometer calibration in cable revolutions per mile, about 1000 for most GM mechanical speedometers and 1001 for many Ford units
- E is the speedometer error as a percent, where a positive value reads high (fast) and a negative value reads low (slow)
- Nd_installed is the driven gear tooth count currently in the transmission
Pick what you want to find at the top of the calculator. In “Driven gear teeth” mode you enter the drive gear teeth, the axle ratio, and the tire size, and the tool returns the driven gear you should fit. Because driven gears only come in whole teeth, it also rounds to the nearest gear and shows the small error left over after rounding, so you can see whether the closest available gear reads slightly high or slightly low. In “Speedometer error” mode you also enter the driven gear that is currently installed, and the tool returns how far off the needle is and the driven gear that would zero it out. The advanced options let you switch the calibration constant or set a reference speed other than 60 mph.
GM Speedometer Gear Tooth Ranges and Sensitivity
Driven gears are grouped by transmission family, and each family covers a fixed tooth range across one or two housings. The last column is the piece most calculators leave out: how much the reading shifts for each tooth you add or remove. That value is roughly 100 divided by the tooth count, so smaller gears move the needle far more per tooth.
| Transmission family | Driven gear teeth | Example colors | Change per tooth |
|---|---|---|---|
| TH350C, Powerglide, Muncie 4-speed | 17 to 25 | Brown 18, Red 21 | about 5% |
| TH350, TH400, 700R4 | 34 to 45 (two housings: 34 to 39, 40 to 45) | White 36, Black 40, Green 42 | about 2.5% |
The drive gear sits on the output shaft and is swapped far less often. These are the counts you will usually see, but confirm yours by counting teeth, since year and build can vary.
| Transmission | Typical drive gear teeth |
|---|---|
| TH350 / TH400 | 8 |
| 700R4 / 4L60 | 13 to 17 |
| Powerglide | 8 |
| Muncie / Saginaw manual | varies, count to confirm |
One more practical point on direction: if the needle reads high, the driven gear has too few teeth, so add teeth. If it reads low, the gear has too many teeth, so remove teeth. A well set up speedometer is usually aimed to read exactly right or a hair high, since a needle that reads low understates how fast you are actually going.
Example Problems
Example 1: Find the driven gear for a tire change.
You run a TH350 with an 8-tooth drive gear, a 3.73 axle, and 28 inch tires. First the tire revolutions per mile: R = 20168 / 28 = 720. Then the driven gear: Nd = (8 * 3.73 * 720) / 1000 = 21.5, which rounds to a 21-tooth driven gear. At 21 teeth the needle reads about 2.3 percent high, so at a true 60 mph it shows about 61 mph.
Example 2: Check a set of gears you already have.
You have a drive gear of 17 teeth, a 3.08 axle, 29 inch tires, and a 40-tooth driven gear fitted. The ideal driven gear is (17 * 3.08 * 695) / 1000 = 36 teeth. Because 40 is four teeth more than 36, the needle reads low: at a true 60 mph the speedometer shows about 55 mph. Dropping to a 36-tooth driven gear brings it back to correct.
Frequently Asked Questions
Do I add or remove teeth if my speedometer reads too fast?
If the speedometer reads high, the driven gear has too few teeth, so you add teeth to slow the cable down. Each added tooth lowers the reading by roughly 2.5 percent on the 34 to 45 tooth gears and by roughly 5 percent on the 17 to 25 tooth gears. Removing a tooth does the opposite and raises the reading.
Where are the drive and driven gears located?
The drive gear is pressed onto the transmission output shaft inside the tailhousing. The driven gear sits in a small removable housing or sleeve on the side of the tailhousing, and the speedometer cable threads onto it. In most recalibrations you only pull and replace the driven gear, which is a quick job, while changing the drive gear means going deeper into the transmission.
Does this calculator work for an electronic speedometer?
No. This method covers cable-driven mechanical speedometers that use a physical drive and driven gear. An electronic speedometer reads a vehicle speed sensor and is corrected by its pulses per mile using a programmer, a recalibration module, or a tune, not by swapping a gear. If your transmission feeds a sensor rather than a cable, use the electronic method instead.
