Gear Ratio Speed Calculator — Vehicle Speed from RPM
Enter engine RPM, transmission gear ratio, final drive ratio and tyre outer diameter to find road speed in km/h, mph and m/s, plus wheel RPM.
RPM
mm
Road speed at the entered RPM, gear ratios and tyre size
- 1
Total drive ratio
3.5 × 3.73 = 13.055 - 2
Wheel speed
3,000 ÷ 13.055 = 229.8 - 3
Tyre circumference
π × 635 ÷ 1000 = 1.9949Outer diameter (mm) converted to metres and multiplied by π. - 4
Vehicle speed (km/h)
229.8 × 1.9949 × 60 ÷ 1000 = 27.5
How does this calculator work?
Wheel RPM = Engine RPM ÷ (Gear Ratio × Final Drive). Speed (km/h) = Wheel RPM × π × Tyre Diameter (m) × 60 ÷ 1000. A 205/55R16 tyre has an outer diameter of about 632 mm. Factory speedometers typically read 2–4% high. Multiply km/h by 0.621 to convert to mph.
Formula
How this is calculated
In a vehicle drivetrain, engine power passes through the transmission (with its selected gear ratio GR) and then through the differential (with its final drive ratio FD). The combined reduction is Total = GR × FD, so Wheel RPM = Engine RPM ÷ Total.
Each wheel revolution advances the car by one tyre circumference (π × D, where D is the outer diameter in metres). Multiplying wheel RPM by circumference and converting to km/h gives: Speed (km/h) = Wheel RPM × π × D × 60 ÷ 1000. The factor 60 converts per-minute to per-hour; dividing by 1000 converts metres to kilometres.
Tyre outer diameter is not printed on the sidewall directly but can be derived from the tyre code. For a 205/55R16 tyre: sidewall height = 205 × 0.55 = 112.75 mm; rim diameter = 16 × 25.4 = 406.4 mm; outer diameter ≈ 406.4 + 2 × 112.75 ≈ 632 mm. The default 635 mm suits most common passenger car fitments. The calculator assumes 100% drivetrain efficiency — practical true speeds are within 1–2% of the result. Factory speedometers are calibrated to read 0–4% high in many markets.
Frequently asked questions
The final drive ratio is the gear reduction in the differential (or transaxle). A ratio of 3.73 means the driveshaft turns 3.73 times per wheel revolution. Combined with the transmission ratio it gives the total reduction from engine to wheel.
From a tyre code such as 205/55R16: sidewall height = 205 × 0.55 = 112.75 mm; rim diameter = 16 × 25.4 = 406.4 mm; outer diameter = 406.4 + 2 × 112.75 ≈ 632 mm. Enter this value in the tyre diameter field.
Factory speedometers are legally required to read at or above true speed in most countries, so they are often calibrated 2–4% high. Tyre wear, pressure changes and non-OEM wheel/tyre combinations also shift the reading. This calculator gives the theoretical true speed from your inputs.
Also known as
TG we-Calculate Editorial Team. (2026). Gear Ratio Speed Calculator — Vehicle Speed from RPM [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/gear-ratio-speed-calculator
TG we-Calculate Editorial Team. "Gear Ratio Speed Calculator — Vehicle Speed from RPM." TG we-Calculate. 2026. https://we-calculate.com/calculator/gear-ratio-speed-calculator.
TG we-Calculate Editorial Team, "Gear Ratio Speed Calculator — Vehicle Speed from RPM," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/gear-ratio-speed-calculator
@misc{wecalculate_gear_ratio_speed_calculator, title = {Gear Ratio Speed Calculator — Vehicle Speed from RPM}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/gear-ratio-speed-calculator}}, year = {2026}, note = {TG we-Calculate} }
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