Mechanical speedometers are driven by a cable from a small plastic gear pair in the transmission tail. Standard speedometers turn 1,001 times per mile, so the driven gear must be matched to the tyre size and axle ratio – otherwise the speedometer and odometer will be off.
How to use the Speedometer Gear Calculator
- Tyre diameter
- Axle ratio
- Drive gear teeth
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the tyre diameter, axle ratio and the number of teeth on the drive (worm) gear. Round the driven gear up or down to the nearest available size.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Tyre diameter | Units: in, cm, mm, ft, m |
| Axle ratio | A number |
| Drive gear teeth | A number |
What each result means
- Tyre revolutions per mile = 20168÷([Tyre diameter]÷0.0254)
- Driven gear teeth (exact) = [Axle ratio]×[Tyre revolutions per mile]×[Drive gear teeth]÷1001
- Driven gear to use = round([Driven gear teeth (exact)])
- Speedometer error with that gear = ([Driven gear teeth (exact)]÷[Driven gear to use] – 1)×100
Example values
How the main result changes when Tyre diameter changes and everything else stays at the example values above:
| Tyre diameter (in) | Driven gear to use |
|---|---|
| 14 | 43 |
| 21 | 29 |
| 28 | 21 |
| 35 | 17 |
| 42 | 14 |
| 56 | 11 |
Formula
Tyre revolutions per mile ≈ 20,168 ÷ tyre diameter (in)Driven gear teeth = axle ratio × revolutions per mile × drive gear teeth ÷ 1,001Worked example
Inputs — Tyre diameter: 28 in; Axle ratio: 3.73; Drive gear teeth: 8.
Results — Tyre revolutions per mile = 720.3; Driven gear teeth (exact) = 21.47; Driven gear to use = 21; Speedometer error with that gear = 2.2 %.
Frequently asked questions
Why 1,001 revolutions per mile?
Standard mechanical speedometers are calibrated so the cable turns 1,001 times per mile.
What if the exact gear doesn't exist?
Pick the nearest size; the error shown tells you how far off the speedometer will read.
Do I need a new drive gear?
Only when the driven gear needed is outside the range available for your drive gear.
What formula does the Speedometer Gear Calculator use?
Tyre revolutions per mile ≈ 20,168 ÷ tyre diameter (in).