dirt bike
Sprocket comparison
Compare rear-sprocket options for a dirt bike and quantify the trade-off between gearing reduction and theoretical road speed at a fixed RPM.
Open calculator
DIRT BIKE CALCULATORDirt Bike Gear Ratio
Dirt Bike Gear Ratio
Calculator
Calculate dirt-bike overall gearing from primary, transmission and sprocket reductions. Compare theoretical speed at your entered engine RPM.
Your setup
teeth
Range: 6.00–250 teethteeth
Range: 6.00–300 teethrpm
Range: 0.00–30,000 rpmin
Range: 1.97–78.74 in:1
Engine to gearbox input; use the actual specification.:1
Range: 0.10–10.00 :1Your results
CalculatedOverall reduction5.40:1
Wheel speed1,111.11rpm
Theoretical speed83.29mph
Metric speed134.04km/h
A larger reduction multiplies ideal wheel torque more and gives lower speed at a fixed engine RPM. Available power determines whether the engine can reach that RPM under load.
READ THE ASSUMPTIONS
No allowance for tire slip, belt slip, tire growth, aerodynamic drag, rolling resistance or engine power. A converter changes ratio during operation; evaluate multiple operating points.
Compare a rear-sprocket change
A larger rear gives more ideal torque multiplication; a smaller rear gives more theoretical speed at equal RPM. Confirm fit, chain length and engine load.
| Rear teeth | Overall ratio | Theoretical MPH | Trade-off |
|---|---|---|---|
| 39 | 4.68:1 | 96.10 | Less reduction; more speed per RPM |
| 45 | 5.40:1 | 83.29 | Your current setup |
| 51 | 6.12:1 | 73.49 | More reduction; more torque multiplication |
THE FORMULA
How the calculation works
Follow the formula and keep the input units consistent at every step.
R = (rear / front) × (jackIn / jackOut) × (jack2In / jack2Out) × converter × primary × gear; v = RPM × π × D / (60 × R)- Divide driven teeth by driving teeth for each chain path.
- Multiply all reductions, including any primary drive, selected gear and converter operating ratio.
- Divide engine RPM by overall reduction for wheel RPM; multiply by circumference and divide by 60 for m/s.
A Worked Example
A 12-tooth driver and 60-tooth axle sprocket give 60 ÷ 12 = 5:1. With direct drive, 3,600 rpm and a 13-inch tire, wheel speed is 720 rpm and theoretical road speed is 27.85 MPH (44.81 KPH).
What Your Result Means
A larger reduction multiplies ideal wheel torque more and gives lower speed at a fixed engine RPM. Available power determines whether the engine can reach that RPM under load.
Assumptions & Limitations
No allowance for tire slip, belt slip, tire growth, aerodynamic drag, rolling resistance or engine power. A converter changes ratio during operation; evaluate multiple operating points.
Common Mistakes
Do not invert driven/driving teeth or omit the primary reduction. On a jackshaft, enter input and output teeth on the same shaft; the full two-chain product is (jackIn/front) × (rear/jackOut).
Questions about this calculation
Which values should I enter?
Divide driven teeth by driving teeth for each chain path. Do not invert driven/driving teeth or omit the primary reduction. On a jackshaft, enter input and output teeth on the same shaft; the full two-chain product is (jackIn/front) × (rear/jackOut).
How should I interpret the result’s precision?
Displayed results are rounded for readability. Intermediate calculations retain floating-point precision; extra displayed decimals do not improve your input accuracy.
Can I use metric units?
Yes. Select Metric above the calculator to convert your inputs. Results always state their units. Tire-size designations keep their conventional mm and inch units.
What are the main limitations?
No allowance for tire slip, belt slip, tire growth, aerodynamic drag, rolling resistance or engine power. A converter changes ratio during operation; evaluate multiple operating points.
Method, sources & relevant guides
Angular speed and tangential speed. Gear reductions follow tooth engagement and conservation of rotation count.
OpenStax — University Physics: rotation with constant angular accelerationMethod documentation updated October 18, 2026. Independent professional review has not been claimed. Read the calculation and review policy.