Road speed is fixed by four things: how fast the engine turns, how much the gearbox and final drive reduce that rotation, and how far the tyre travels in one turn. Multiply the wheel speed by the rolling circumference and you have distance per minute, which is a speed. Everything else is unit handling.
The wheel turns at engine rpm divided by the overall ratio, which is the gearbox ratio multiplied by the final drive. With a 0.62 top gear and a 3.94 final drive the overall ratio is 2.4428, so at 3,000 rpm the wheels turn 1228.1 times a minute. A 632 mm tyre has a circumference of 1.9855 metres, giving 2438 metres a minute.
Multiplying by 60 and dividing by 1000 turns metres per minute into kilometres per hour, 146.3 km/h here, and dividing by 1.609344 gives 90.9 mph. Running the sum backwards, cruising at 70 mph in that gear means 2,310 rpm. Tall gearing lowers cruising revs and noise but leaves less torque multiplication for overtaking.
The formula
rpm- Engine speed in revolutions per minute
gearbox ratio- Ratio of the selected gear, under 1.00 for an overdrive top
final drive- Differential ratio, typically 3.0 to 4.5
circumference- π × tyre diameter, the distance covered in one wheel revolution
How to use this calculator
- Enter the engine speed you want to check, and the redline for the top-speed figure.
- Enter the gearbox ratio for the gear you are interested in and the final drive ratio.
- Enter the tyre rolling diameter in millimetres, or switch the selector to enter circumference.
- The gauge shows road speed in mph; km/h, redline speed and rpm at 70 sit below.
Worked examples
3,000 rpm in a 0.62:1 top gear
Overall ratio is 0.62 x 3.94 = 2.4428. The wheels turn 1228.1 rpm, and a 632 mm tyre covers 1.9855 m per turn, so the car travels 2438 m per minute — 146.3 km/h or 90.9 mph.
What a shorter final drive does
Swapping to a 4.4 final drive raises the overall ratio to 2.7280, so the same 3,000 rpm now gives only 81.4 mph. Cruising at 70 mph would need 2,580 rpm — livelier acceleration, noisier motorway.
How to read your score
Frequently asked questions
Where do I find my gearbox and final drive ratios?
The workshop manual or the manufacturer technical specification lists them gear by gear. Enthusiast forums for a specific model usually have them too. Automatic gearboxes have a torque converter that slips below lock-up, so the calculated speed is only exact once the converter is locked.
Does tyre slip make the answer wrong?
Slightly. A driven tyre deforms under torque and the effective rolling radius is a little smaller than the free radius, so real speed is typically one to two per cent below the geometric figure under load. It is negligible at steady cruise.
What rolling diameter should I enter?
The loaded rolling diameter, which is a little under the nominal geometric diameter because the tyre squashes at the contact patch. Using the nominal figure from a tyre size calculation is close enough for gearing work.
Why does my speedometer disagree?
Speedometers are legally allowed to over-read and are deliberately calibrated optimistic — UK rules permit a reading up to 10 per cent plus 6.25 mph above true speed, and never below it. A calculated speed a few per cent below the indicated one is entirely normal.
Can I use this to check a gearing change?
Yes, and it is the main use. Run the calculation with the current ratio and tyre, then again with the proposed ones, and compare the rpm at 70 mph. That single number tells you most of what a gearing change will feel like.
Gearing reference
| Gear | Overall ratio | 2000 rpm | 3000 rpm | 4000 rpm | rpm at 70 mph |
|---|---|---|---|---|---|
| 1st | 14.854:1 | 10.0 mph | 15.0 mph | 19.9 mph | 14,046 |
| 2nd | 8.038:1 | 18.4 mph | 27.6 mph | 36.8 mph | 7,601 |
| 3rd | 5.201:1 | 28.5 mph | 42.7 mph | 56.9 mph | 4,918 |
| 4th | 3.861:1 | 38.3 mph | 57.5 mph | 76.7 mph | 3,651 |
| 5th | 2.994:1 | 49.4 mph | 74.2 mph | 98.9 mph | 2,832 |
| 6th | 2.443:1 | 60.6 mph | 90.9 mph | 121.2 mph | 2,310 |
Tyre circumference taken as 1.9855 m. Substitute your own ratios and tyre size in the inputs above.