Quick answer
Driven gear teeth = drive teeth × axle ratio × tire revolutions per mile ÷ 1,001. With an 8-tooth drive gear, a 3.73 axle and a 30-inch tire the unrounded answer is 20.04 teeth, so a 20-tooth gear is recommended.
Enter the speedometer drive gear's teeth, the axle ratio and the tire diameter. The calculator returns the driven gear teeth, the unrounded value and the reading error after rounding.
Related tools: Gear Ratio Calculator: Gear ratio, output speed and torque multiplication. Gear Speed Calculator: Output speed and ratio of a gear pair. Transmission Gear Ratio Calculator: Vehicle speed from engine rpm, gear ratio, final drive and tire size.
How to use the Speedometer Gear Calculator
- Enter teeth on the speedometer drive gear (a whole number, between 1 and 60).
- Enter axle ratio (for example 3.73) (between 1 and 8).
- Enter tire diameter (1 or more).
- Choose tire diameter unit: Inches, Millimetres and Centimetres.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Teeth on the speedometer drive gear | A whole number | ≥ 1, ≤ 60 |
| Axle ratio (for example 3.73) | A number | ≥ 1, ≤ 8 |
| Tire diameter | A number | ≥ 1 |
| Tire diameter unit | Choose one: Inches; Millimetres; Centimetres | Starts at Inches |
Formula and method
Formula
tire revolutions per mile = 63,360 ÷ (π × diameter in inches); driven teeth = drive teeth × axle ratio × revolutions per mile ÷ 1,001
IEEE-754 double precision; shown to at most 6 decimal places.
The traditional formula for mechanical (cable) speedometers: the cable turns 1,001 times per mile on the original design, so the driven gear must have drive teeth × axle ratio × tire revolutions per mile ÷ 1,001 teeth. Use the loaded rolling diameter (or revolutions per mile from the tire maker) for best accuracy. The answer is rounded to the nearest whole tooth, so the speedometer can still read a little fast or slow; the result also shows the resulting speedometer error for the rounded tooth count. Electronic speedometers are recalibrated electronically instead.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Teeth on the speedometer drive gear = 8; Axle ratio (for example 3.73) = 3.73; Tire diameter = 30; Tire diameter unit = Inches.
| Result | Value |
|---|---|
| Driven gear teeth | 20 teeth |
| Unrounded | 20.04 |
| Tire revolutions per mile | 672.3 |
| Speedometer reading error after rounding | -0.20% |
Example 2
Inputs: Teeth on the speedometer drive gear = 10; Axle ratio (for example 3.73) = 3.08; Tire diameter = 28.5; Tire diameter unit = Inches.
| Result | Value |
|---|---|
| Driven gear teeth | 22 teeth |
| Unrounded | 21.77 |
| Tire revolutions per mile | 707.7 |
| Speedometer reading error after rounding | 1.04% |
Example 3
Inputs: Teeth on the speedometer drive gear = 7; Axle ratio (for example 3.73) = 4.1; Tire diameter = 762; Tire diameter unit = Millimetres.
| Result | Value |
|---|---|
| Driven gear teeth | 19 teeth |
| Unrounded | 19.27 |
| Tire revolutions per mile | 672.3 |
| Speedometer reading error after rounding | -1.43% |
The first line is the headline result (driven gear teeth). Below it the speedometer Gear Calculator lists unrounded, tire revolutions per mile and speedometer reading error after rounding, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
The two steps
Tire revolutions per mile = 63,360 ÷ (π × diameter in inches); for a 30-inch tire that is 672.3. Then driven teeth = 8 × 3.73 × 672.3 ÷ 1,001 = 20.04. The constant 1,001 comes from the standard speedometer cable convention of 1,001 revolutions per mile at the cable.
Rounding error
Gears have whole teeth, so the answer is rounded and the speedometer reads slightly fast or slow. Rounding 20.04 to 20 leaves an error of about 0.2%, which the calculator reports. Check against a GPS reading after any change.
How the result changes with teeth on the speedometer drive gear
Holding every other input at the first worked example, this table changes only teeth on the speedometer drive gear. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Teeth on the speedometer drive gear | Driven gear teeth | Unrounded | Tire revolutions per mile |
|---|---|---|---|
| 4 | 10 teeth | 10.02 | 672.3 |
| 8 | 20 teeth | 20.04 | 672.3 |
| 16 | 40 teeth | 40.08 | 672.3 |
| 32 | 80 teeth | 80.16 | 672.3 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Teeth on the speedometer drive gear: left empty shows "Enter teeth on the speedometer drive gear." beside that field and no result.
- Axle ratio (for example 3.73): entering "0.5" shows "Axle ratio (for example 3.73) must be at least 1." beside that field and no result.
- Tire diameter: entering "0" shows "Tire diameter must be at least 1." beside that field and no result.
Choosing between the Speedometer Gear Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Gear Ratio Calculator](/engineering/gear-ratio-calculator/) | driving gear teeth and driven gear teeth | Gear ratio |
| [Gear Speed Calculator](/engineering/gear-speed-calculator/) | driver gear teeth, driven gear teeth and input speed | Output speed |
| [Transmission Gear Ratio Calculator](/engineering/transmission-gear-ratio-calculator/) | transmission gear ratio, final drive ratio and tire diameter | Vehicle speed |
| [Ohm's Law Calculator](/engineering/ohm-s-law-calculator/) | Current | |
| [Voltage Drop Calculator](/engineering/voltage-drop-calculator/) | current, one-way length and wire gauge | Voltage drop |
Assumptions and limitations
- Rounds to a whole number of teeth
- Mechanical cable speedometers only
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for teeth on the speedometer drive gear, axle ratio (for example 3.73) and tire diameter to confirm this.
Frequently asked questions
When do I need to recalibrate?
After changing the axle ratio or tire diameter.
Does it work with electronic speedometers?
No, it is for mechanical drive/driven gear sets; electronic units are recalibrated in software.
Where do I find the axle ratio?
On the axle tag, the build sheet, or by counting driveshaft turns per wheel turn.
What does the Speedometer Gear Calculator calculate?
Number of teeth for the speedometer driven gear after a tire or axle ratio change. It returns Driven gear teeth, Unrounded, Tire revolutions per mile and Speedometer reading error after rounding.
What formula does the Speedometer Gear Calculator use?
tire revolutions per mile = 63,360 ÷ (π × diameter in inches); driven teeth = drive teeth × axle ratio × revolutions per mile ÷ 1,001. The traditional formula for mechanical (cable) speedometers: the cable turns 1,001 times per mile on the original design, so the driven gear must have drive teeth × axle ratio × tire revolutions per mile ÷ 1,001 teeth.
What do I need to enter in the speedometer Gear Calculator?
You enter Teeth on the speedometer drive gear, Axle ratio (for example 3.73), Tire diameter and Tire diameter unit.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Rounds to a whole number of teeth.
What are the limits of this calculator?
Rounds to a whole number of teeth. Mechanical cable speedometers only.
What does the “Tire diameter unit” option change?
It sets how the speedometer Gear is worked out. The choices are Inches, Millimetres and Centimetres.
What is an example of a speedometer Gear result?
With teeth on the speedometer drive gear 8, axle ratio (for example 3.73) 3.73, tire diameter 30 and tire diameter unit Inches, the driven gear teeth is 20 teeth.
Where does the method come from?
The method follows: Standard mechanical-speedometer gearing formula (cable revolutions per mile of 1,001) used in automotive references; the rolling circumference is exact geometry: 1 mile = 63,360 in.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Rounds to a whole number of teeth.
Related calculators
- Gear Ratio Calculator — Gear ratio, output speed and torque multiplication.
- Gear Speed Calculator — Output speed and ratio of a gear pair.
- Transmission Gear Ratio Calculator — Vehicle speed from engine rpm, gear ratio, final drive and tire size.
- Ohm's Law Calculator — Solve voltage, current or resistance.
- Power-to-Weight Ratio Calculator — Power per unit weight of a vehicle.
Sources
- Standard mechanical-speedometer gearing formula (cable revolutions per mile of 1,001) used in automotive references; the rolling circumference is exact geometry: 1 mile = 63,360 in
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.