Gear Ratio Calculator
Find road speed at any engine speed, or the revs at a given speed.
What the Gear Ratio Calculator does
Road speed follows directly from engine speed once you know how much the drivetrain divides it and how far the tyre travels per revolution. Gear ratio and final drive multiply together, and the tyre’s circumference converts wheel revolutions into distance.
Formula
Overall ratio = Gear ratio × Final driveMPH = RPM × Tyre diameter ÷ (Overall ratio × 336.135)RPM = MPH × Overall ratio × 336.135 ÷ Tyre diameterRevolutions per mile = 63,360 ÷ (π × Diameter)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Solve for | one of 2 options | Yes | — |
| Engine speed | rpm | In some modes | Accepts more than 0. Shown Solve for is Road speed from engine speed and Tyre specified by is Tyre size code, or Solve for is Road speed from engine speed and Tyre specified by is Overall diameter. |
| Road speed | number | In some modes | Accepts more than 0. Shown Solve for is Engine speed from road speed and Tyre specified by is Tyre size code, or Solve for is Engine speed from road speed and Tyre specified by is Overall diameter. |
| Distance unit | one of 2 options | Yes | — |
| Transmission gear ratio | number | Yes | Accepts more than 0. |
| Final drive ratio | number | Yes | Accepts more than 0. |
| Tyre specified by | one of 2 options | Yes | — |
| Tyre size | text | In some modes | Shown Solve for is Road speed from engine speed and Tyre specified by is Tyre size code, or Solve for is Engine speed from road speed and Tyre specified by is Tyre size code. |
| Tyre diameter | inches | In some modes | Accepts more than 0. Shown Solve for is Road speed from engine speed and Tyre specified by is Overall diameter, or Solve for is Engine speed from road speed and Tyre specified by is Overall diameter. |
How to use it
- Choose Solve for and Distance unit.
- Enter Transmission gear ratio and Final drive ratio.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
3,000 rpm in a 1:1 gear with a 3.73 final drive on 26 inch tyres.
- RPM
- 3,000
- Gear
- 1.00
- Final
- 3.73
- Tyre
- 26 in
3,000 × 26 ÷ (3.73 × 336.135) = 62.2 mph, or 100.1 km/h.
Frequently asked questions
What does a higher final drive ratio do?
A numerically higher ratio multiplies torque more, improving acceleration, but raises engine speed at any given road speed and costs fuel economy on the motorway.
How do bigger tyres change things?
A taller tyre travels further per revolution, lowering engine revs at a given speed — effectively a taller gear — and it makes the speedometer under-read.