Mechanical Speedometer Gear Calculator
Enter the tire, axle, and transmission drive-gear details to calculate a mechanical speedometer driven gear. The comparison shows why the nearest tooth count is not always the gear with the smallest reading error.
Best whole-tooth driven gear21
- Calculated tire revolutions per mile
- 720.29
- Ideal driven-gear teeth
- 21.47
- Neighboring gear comparison
- 21 teeth: +2.25% (reads fast); 22 teeth: -2.40% (reads slow)
- Evaluated gear error
- 2.25%
- Indicated speed at 60 mph actual
- 61.35
- Actual speed at 60 mph indicated
- 58.68
A positive error means the speedometer reads fast. The recommendation compares only the two adjacent whole-tooth counts; confirm that the gear and housing fit the transmission.
How to use this calculator
- Choose published tire revolutions per mile when the tire maker provides it; otherwise enter the loaded tire diameter.
- Enter the rear axle ratio and count the teeth on the transmission speedometer drive gear.
- Choose the cable-revolutions-per-mile constant specified for the speedometer.
- Enter an installed or available driven gear to see how far the speedometer will read fast or slow.
How the driven gear is calculated
A conventional cable-driven speedometer is calibrated for a known number of cable revolutions per mile. The transmission output turns the small drive gear, the driven gear turns the cable, and the rear axle ratio and tire rolling circumference determine how quickly that happens on the road.
When tire diameter is the available measurement, the calculator first estimates tire revolutions per mile as R = 63,360 ÷ (π × D). Here, D is the loaded diameter in inches and 63,360 is the number of inches in a mile. A tire maker’s published revolutions-per-mile figure is normally more useful because it represents rolling circumference rather than a static measurement.
The ideal driven-gear tooth count is:
T = (Nd × A × R) ÷ C
- Nd is the tooth count on the transmission drive gear.
- A is the rear axle ratio.
- R is tire revolutions per mile.
- C is the speedometer calibration, commonly 1,000 or 1,001 cable revolutions per mile.
Choosing between whole-tooth gears
Gears come in whole teeth, so the ideal result may fall between available counts. For each neighboring candidate Ng, the calculator uses Q = T ÷ Ng for the indicated-to-actual speed ratio and E = 100 × (Q − 1) for the signed percentage error. A positive error means the speedometer reads fast; a negative error means it reads slow.
This matters because ordinary rounding only compares distances from the ideal tooth count. The speed error is a ratio, so the lower and upper candidates are not always equally wrong. The tool calculates both values and recommends the smaller absolute percentage error; in an exact tie, it keeps the lower count. It also translates the selected gear into indicated speed at 60 mph actual and actual speed at 60 mph indicated.
What moves the result and what it leaves out
Tire revolutions per mile and axle ratio change the required driven-gear count directly. More tire revolutions per mile, a numerically higher axle ratio, or more drive-gear teeth all require more driven-gear teeth. A larger driven gear turns the cable more slowly, so it makes the speedometer read slower.
This is a calibration calculation, not a parts-fitment catalog. Tire pressure, tread wear, load, and speed can change rolling radius. It does not identify the correct gear pitch, color, shaft style, driven-gear housing, adapter ratio, or whether a transmission offers the calculated count. Electronic or computer-controlled speedometers need their manufacturer’s calibration procedure instead.
Worked example
With an 8-tooth drive gear, a 3.73 axle ratio, tires rated at 720 revolutions per mile, and a 1,001 rev/mi speedometer, the ideal driven gear is (8 × 3.73 × 720) ÷ 1,001 = 21.46 teeth. A 21-tooth gear reads 2.21% fast, while a 22-tooth gear reads 2.44% slow, so the calculator recommends 21 teeth.
If the 21-tooth gear is installed, the speedometer indicates 61.32 mph at 60 mph actual. Conversely, at 60 mph indicated, the actual road speed is 58.70 mph.
Common questions
Should I use tire diameter or tire revolutions per mile?
Use the tire manufacturer’s published revolutions-per-mile specification when it is available. It is based on rolling circumference and is generally closer to the tire’s behavior on the road than a nominal or unloaded diameter. Measure loaded diameter only when a published value is unavailable.
Why can ordinary rounding choose the worse speedometer gear?
The tooth count is rounded, but speedometer error depends on the ratio of ideal teeth to installed teeth. Counts equally close to the ideal can create slightly different percentage errors. Comparing both adjacent gears directly identifies the smaller error.
What is the difference between the drive gear and driven gear?
The drive gear is attached to the transmission output shaft, while the driven gear meshes with it and turns the speedometer cable. Changing the driven gear is often the practical adjustment, but some transmissions also use different drive gears or adapters for larger corrections.
Why do some formulas divide by 1000 and others by 1001?
The divisor is the cable revolutions per mile that the speedometer expects. Many mechanical instruments are nominally 1,000 rev/mi, while some aftermarket formulas use 1,001. Use the value specified for the instrument or existing calibration system rather than treating the constants as interchangeable.
What if the calculated tooth count is unavailable?
Compare the available adjacent counts and choose the one with the error you can accept, after confirming it fits the housing. If the error remains large, the transmission may need a different drive gear or a ratio adapter. Verify the finished installation over a measured mile or against a reliable GPS reference.