Gear Ratio Calculator
Estimate theoretical road speed from engine RPM, gear ratio, final drive and tire diameter. Actual speed can differ because of tire rolling radius, drivetrain behavior and other conditions.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
About this calculator
Estimate theoretical road speed from engine RPM, gear ratio, final drive and tire diameter. Actual speed can differ because of tire rolling radius, drivetrain behavior and other conditions.
Worked example
calculated with the default values below
Engine speed 3,000 rpm · Gear ratio 1 :1 · Final drive ratio 3.73 :1 · Tyre diameter 26 in Overall ratio 3.730 :1 · Wheel speed 804.3 rpm · Road speed 62.2 mph · Road speed 100.1 km/h · Speed per 1000 rpm 20.74 mph
Method and assumptions
R = G x F; wheelRpm = rpm/R; mph = wheelRpm x π x D/12 x 60/5280; kph = mph x 1.609344 Tire circumference is pi x D/12 feet; mph = wheel rpm x circumference x 60/5280; mph per 1000 rpm = mph / (rpm/1000). Estimate theoretical road speed from engine RPM, gear ratio, final drive and tire diameter. Actual speed can differ because of tire rolling radius, drivetrain behavior and other conditions. The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
Data sources
The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
Disclaimer
These calculators give estimates for general information only. Real fuel consumption, performance and range vary with driving style, load, terrain, temperature and vehicle condition. Do not rely on these results for towing limits, payload or braking decisions — use the rating plate of your vehicle.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
R = G x F; wheelRpm = rpm/R; mph = wheelRpm x π x D/12 x 60/5280; kph = mph x 1.609344
- G
- transmission gear ratio
- F
- final-drive ratio
- R
- overall ratio
- rpm
- engine speed (rpm)
- D
- tire diameter (in)
- mph
- vehicle speed (mph)
Tire circumference is pi x D/12 feet; mph = wheel rpm x circumference x 60/5280; mph per 1000 rpm = mph / (rpm/1000).
Frequently asked questions
How is theoretical road speed calculated?
The calculator applies the gear ratio and final-drive ratio to engine RPM, then multiplies wheel RPM by tire circumference. It assumes no drivetrain slip and uses the entered tire diameter.
Why can actual speed differ?
Loaded tire radius, tire growth, drivetrain behavior, transmission choice and other conditions affect road speed. This estimate does not calculate speedometer error or road load.
Which inputs and limits apply to Gear Ratio Calculator?
Use the units shown beside each field. These are form limits, not recommended settings. Engine speed: 100–12,000 rpm; Gear ratio: 0.1–10 :1; Final drive ratio: 0.1–10 :1; Tyre diameter: 5–60 in R = G x F; wheelRpm = rpm/R; mph = wheelRpm x π x D/12 x 60/5280; kph = mph x 1.609344 Tire circumference is pi x D/12 feet; mph = wheel rpm x circumference x 60/5280; mph per 1000 rpm = mph / (rpm/1000). Estimate theoretical road speed from engine RPM, gear ratio, final drive and tire diameter. Actual speed can differ because of tire rolling radius, drivetrain behavior and other conditions. The references explain definitions and methods; any extra preset must be checked against the assumptions on this page.
What is a complete worked example for Engine speed = 3,000 rpm; Gear ratio = 1 :1?
Inputs: Engine speed = 3,000 rpm; Gear ratio = 1 :1; Final drive ratio = 3.73 :1; Tyre diameter = 26 in. Calculation: R = G x F; wheelRpm = rpm/R; mph = wheelRpm x π x D/12 x 60/5280; kph = mph x 1.609344. Results: Overall ratio = 3.730 :1; Wheel speed = 804.3 rpm; Road speed = 62.2 mph; Road speed = 100.1 km/h; Speed per 1000 rpm = 20.74 mph.