dirt bike sprocket calculator formula

The formula to calculate the final drive ratio for a dirt bike using the sprocket sizes is:Final Drive Ratio=Rear Sprocket TeethFront Sprocket Teeth\text{Final Drive Ratio} = \frac{\text{Rear Sprocket Teeth}}{\text{Front Sprocket Teeth}}Final Drive Ratio=Front Sprocket TeethRear Sprocket Teeth​

Example:

• Front sprocket (countershaft): 13 teeth
• Rear sprocket: 50 teeth

5013=3.85\frac{50}{13} = 3.851350​=3.85

This means the rear wheel rotates once for every 3.85 rotations of the engine’s front sprocket.

Speed Calculation:

To estimate speed at a given RPM, use:Speed (mph)=RPM×Tire Circumference×60Final Drive Ratio×Primary Ratio×Transmission Ratio×63360\text{Speed (mph)} = \frac{\text{RPM} \times \text{Tire Circumference} \times 60}{\text{Final Drive Ratio} \times \text{Primary Ratio} \times \text{Transmission Ratio} \times 63360}Speed (mph)=Final Drive Ratio×Primary Ratio×Transmission Ratio×63360RPM×Tire Circumference×60​

where:

• Tire Circumference = π×diameter of the wheel (in inches)\pi \times \text{diameter of the wheel (in inches)}π×diameter of the wheel (in inches)
• Primary Ratio = Ratio inside the engine (varies by model)
• Transmission Ratio = Gear ratio for the selected gear
• 63360 = Converts inches per minute to miles per hour