How to use it
- Enter the Tyre size, for example 205/55 R16.
- Type the Gear ratios, one per line, from first gear to top gear. The gearbox specification lists them.
- Enter the Final drive ratio and the Engine speed, rpm for the table.
- Read the speed in each gear, and the Engine speed in top gear, rpm for the speed you need.
How the road speed is calculated
The engine turns the gearbox, and the gear ratio and the final drive divide the engine turns down to the wheel turns. Each wheel turn moves the car by its circumference, so the road speed is the engine speed times the circumference, divided by the two ratios. The Wikipedia gear train article gives the same relation in inches per minute, which becomes miles per hour when divided by 1,056.
Speed (km/h) = rpm × circumference (m) × 60 ÷ (1,000 × gear ratio × final drive)
Circumference (m) = π × overall diameter (mm) ÷ 1,000
Rpm for a speed = speed (km/h) × 1,000 × gear ratio × final drive ÷ (60 × circumference)The overall diameter of the tyre
The overall diameter comes from the tyre size: the rim plus two sidewalls, where each sidewall is the section width times the aspect ratio. The Wikipedia article on tyre codes notes that this is a theoretical diameter; the actual diameter is in the tyre maker's data.
What this calculator does not include
- Wheel slip, clutch slip and the change of the rolling radius under load.
- Transmissions without fixed ratios, such as continuously variable gearboxes.
- The engine's speed limit: the table shows what each gear would give, not whether the engine can reach that speed.
Worked example: 205/55 R16, final drive 3.94, 3,000 rpm
Circumference: π × 631.9 mm = 1,985 mm, which is 1.985 m.
First gear 3.615: 3,000 × 1.985 × 60 ÷ (1,000 × 3.615 × 3.94) = 357.3 ÷ 14.24 = 25.1 km/h. Top gear 0.767: 357.3 ÷ (0.767 × 3.94) = 357.3 ÷ 3.022 = 118.2 km/h.
For 100 km/h in top gear the engine needs 100 × 1,000 × 0.767 × 3.94 ÷ (60 × 1.985) = 2,537 rpm.
| Gear | Ratio | km/h at 3,000 rpm |
|---|---|---|
| 1 | 3.615 | 25.1 |
| 2 | 2.188 | 41.5 |
| 3 | 1.541 | 58.9 |
| 4 | 1.213 | 74.8 |
| 5 | 1.000 | 90.7 |
| 6 | 0.767 | 118.2 |
Frequently asked questions
Where do I find the gear ratios of my car?
They are in the gearbox or transmission specification, often in the workshop manual or the maker's technical data. Enter them from first gear to top gear, one per line, with a dot as the decimal point.
What is the final drive ratio?
It is the ratio of the differential: the number of turns of the driveshaft for each turn of the wheels. It is listed with the gear ratios. Enter it as a plain number.
Why does the speed in a gear change with the tyre size?
The circumference sets how far the wheel travels per turn. A bigger tyre travels further per turn, so every gear gives a higher road speed at the same engine speed.
Is the speed in the table what my speedometer shows?
No. It is the true speed worked out from the tyre size. The speedometer uses its own calibration, and a different tyre size makes it read differently; the tyre size comparison shows by how much.
Why does the top-gear rpm differ between cars?
Because the gear ratios, the final drive and the tyre differ. A higher final drive ratio raises the rpm for a given speed, and a larger tyre lowers it.
Does the calculator keep the numbers I type?
No. The calculation runs in your browser, and nothing you enter is sent to a server or saved.
Sources
- Wikipedia: Gear train (vehicle speed from engine speed, tyre circumference and gear ratios; mph by dividing by 1,056)
- Wikipedia: Tire code (ISO metric and inch sizes; the overall diameter formula)
Figures and text last checked on Tuesday, November 10, 2026.