Gear Ratio Speed Calculator
Enter engine RPM, gear ratio, final drive and tire diameter to see your road speed in km/h and mph.
Result
How it is calculated
Wheel speed is the engine speed divided by the two reduction ratios: wheel rpm = rpm ÷ (gear × final drive). The tire's rolling circumference is C = π × D × 0.0254 metres, where D is the overall tire diameter in inches. Road speed is km/h = wheel rpm × C × 60 ÷ 1000, and mph is that figure divided by 1.609344. The tool uses the overall tire diameter, not the rim size, and assumes no wheel slip, no clutch or torque-converter slip and a tire that keeps its full diameter at speed. All four inputs must be above zero, an engine speed above 30,000 rpm or a tire diameter above 160 inches gives no result. The gear is the ratio of the gear you are in, such as 1.00 for a direct top gear or about 3.5 for first.
Worked example
Engine at 3,000 rpm in a 1.00 gear, with a 3.73 final drive and a 26 inch tire. Wheel speed: 3000 ÷ (1 × 3.73) = 804.3 rpm. Circumference: π × 26 × 0.0254 = 2.075 m. Speed: 804.3 × 2.075 × 60 ÷ 1000 = 100.12 km/h. In mph: 100.12 ÷ 1.609344 = 62.21. Result: 100.12 km/h (62.21 mph), with the wheels turning at 804 rpm.
Common mistakes
- Entering the rim size instead of the overall tire diameter. A 15 inch rim with the tire fitted is roughly 25 to 27 inches overall, so the speed comes out far too low.
- Typing the tire diameter in centimetres or millimetres. The field is inches, so 66 for a 26 inch tire gives a speed more than twice too high, and anything above 160 gives no result.
- Entering the ratio the wrong way round, such as 0.27 instead of 3.73. A higher number is a shorter gear and a lower road speed at the same rpm.
- Forgetting the final drive. The gear ratio alone is only part of the reduction, and leaving the final drive at 1 gives a speed several times too high.
- Treating the figure as the speedometer reading. It is the true speed from the geometry only; slip, tire wear and pressure change the real value.
About this calculator
Road speed at a given engine RPM comes down to how many times the wheels turn for each engine revolution, which is set by the gear ratio and final drive ratio together, and how far the car moves for each wheel turn, which is set by the tire’s rolling circumference (π × tire diameter). The full relationship is roughly speed = (engine RPM × tire circumference × 60) / (gear ratio × final drive ratio × unit conversion factor) — a taller gear (numerically lower ratio) or a larger tire both push the same RPM to a higher road speed, while a shorter (numerically higher) gear ratio does the opposite.
Racers, off-roaders and drivetrain tuners use this math when swapping gears, final drive ratios or tire sizes, because changing any one of those three variables shifts the RPM a car sits at for a given speed — critical for keeping an engine in its powerband, for correcting speedometer accuracy after a tire size change, or for balancing acceleration against top speed and fuel economy.
This calculator takes your engine RPM, gear ratio, final drive ratio and tire diameter, and returns your road speed in both km/h and mph.
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