Estimate output shaft RPM and vehicle/wheel speed from gear teeth counts, input RPM, and wheel diameter.
What a Gear Speed Calculator Tells You
A gear speed calculator helps you translate rotating input speed (RPM) into output speed after gearing. If you know how many teeth are on the driver and driven gears, plus the wheel size, you can quickly estimate final RPM and road speed. This is useful for bicycles, go-karts, robotics projects, conveyors, and custom drivetrain setups.
How the Calculation Works
1) Gear ratio from tooth count
For a simple two-gear pair:
- Gear ratio = Driven teeth / Driver teeth
If the driven gear has more teeth than the driver, output RPM decreases and torque increases. If the driven gear has fewer teeth, output RPM increases and torque decreases.
2) Overall reduction including extra gearbox stages
If you have another reduction stage (for example, a transmission or planetary gearbox), multiply it with the primary gear ratio:
- Overall ratio = (Driven / Driver) × Extra gearbox ratio
3) Output RPM and linear speed
- Output RPM = Input RPM / Overall ratio
- Wheel circumference = π × wheel diameter
- Linear speed = Output RPM × circumference / 60
The calculator automatically converts the result into miles per hour and kilometers per hour.
Example
Suppose you have a 12-tooth driver, a 36-tooth driven gear, 3000 input RPM, and a 26-inch wheel:
- Gear ratio = 36 / 12 = 3.00
- Output RPM = 3000 / 3.00 = 1000 RPM
- With a 26-inch wheel, that becomes a practical speed estimate in mph and km/h
This quick math helps when selecting sprockets before buying parts or machining components.
Why Gear Speed Matters
- Performance tuning: Balance acceleration versus top speed.
- Motor safety: Avoid over-speeding a motor or drivetrain element.
- Efficiency: Keep the system near an efficient RPM band.
- Design planning: Compare multiple gear combinations quickly.
Practical Tips for Better Results
Account for real-world losses
Actual speed can be lower due to tire slip, chain/belt losses, bearing drag, wind resistance, and load. Treat calculator output as an ideal baseline, then validate with measured data.
Use accurate wheel diameter
Nominal tire size is often different from rolling diameter under load. If precision matters, measure the true rollout distance for one wheel revolution.
Match gearing to your objective
- Need stronger launch or hill climbing? Increase overall reduction.
- Need higher top speed? Reduce overall reduction (within motor limits).
Common Applications
- Electric bikes and custom bicycle drivetrains
- RC cars and robotics gear trains
- Go-karts and minibikes
- Industrial rollers and conveyor systems
- Prototyping mechanical systems in engineering classes
Frequently Asked Questions
Does this work for chain and belt drives?
Yes. For chain and timing belt setups, tooth count ratio works the same way as simple spur gear ratio for speed estimation.
Can I use this for multi-stage gear trains?
Yes. Multiply each stage ratio together and enter the product in the additional gearbox ratio field.
Is this a torque calculator too?
Not directly. But in ideal conditions, torque scales approximately with overall reduction (ignoring efficiency losses), while RPM scales inversely.