Gear Ratio & Speed Calculator
Use this calculator for cars, motorcycles, go-karts, and even custom drivetrain projects. Enter your gear teeth, RPM, and tire diameter to estimate road speed.
What is a gear ratio?
A gear ratio describes how many turns the input gear makes compared with the output gear. If your driving gear has 14 teeth and the driven gear has 42 teeth, the gear ratio is 42 ÷ 14 = 3.00. That means the input must turn three times for one output turn.
In practical terms, a higher numerical ratio gives more torque multiplication and stronger acceleration, but lower top speed at a given RPM. A lower numerical ratio reduces torque multiplication but can increase top speed.
How speed is calculated from RPM
Once you know the full drivetrain ratio, speed is straightforward to estimate.
The number 63,360 is inches per mile. Tire circumference is π × tire diameter.
How to use this calculator
1) Enter your tooth counts
Input the number of teeth on the driving and driven gears. This is your basic mechanical ratio.
2) Add transmission and final drive values
For direct drive setups, use 1.00. For vehicles, enter the selected transmission gear ratio (for example, 0.78 in overdrive) and axle/final drive ratio (for example, 3.55 or 4.10).
3) Enter RPM and tire diameter
Engine RPM is your current or planned operating point. Tire diameter should be loaded diameter in inches if possible for best accuracy.
4) Click calculate
You’ll get:
- Gear ratio and overall ratio
- Wheel RPM
- Estimated speed in mph and km/h
- Speed per 1,000 RPM
- Required engine RPM for an optional target speed
Example setup
Suppose your drivetrain has these values:
- Driving gear: 14 teeth
- Driven gear: 42 teeth
- Transmission ratio: 1.00
- Final drive ratio: 3.73
- Engine speed: 3,000 RPM
- Tire diameter: 26.5 inches
This creates a large overall reduction, so wheel RPM is much lower than engine RPM, giving strong pulling power but modest road speed.
Choosing the right ratio for your goal
For better acceleration
- Increase overall ratio (higher numerical value)
- Use smaller driving gear or larger driven gear
- Consider a higher final drive ratio
For higher cruising speed
- Reduce overall ratio (lower numerical value)
- Use larger driving gear or smaller driven gear
- Use taller transmission gears and/or larger tires
Common mistakes to avoid
- Mixing units: tire diameter should match the formula assumptions (inches in this calculator).
- Using nominal tire size: real loaded tire diameter can differ from advertised size.
- Ignoring converter or clutch slip: automatic transmissions and CVTs may not be fully locked.
- Assuming perfect real-world speed: aerodynamic drag, rolling resistance, and drivetrain losses matter.
Where this is useful
This calculator is useful for gearing changes on race cars, off-road builds, kart tuning, EV conversion planning, and any project where you want to predict road speed at specific RPM points.
Quick FAQ
Does a bigger tire increase speed?
Yes. A larger diameter tire travels farther per wheel revolution, so speed increases at the same wheel RPM.
Why does my real GPS speed differ?
Because real vehicles have slip, deformation, and drivetrain losses. The calculator provides a theoretical estimate.
Can I use this for bicycles?
Yes. Set transmission and final drive to 1.00, then enter front and rear sprocket teeth, cadence-based RPM (crank RPM), and wheel diameter in inches.