Has a tire size change or rear axle ratio upgrade thrown your speedometer off? Our Speedometer Gear Calculator helps you find the precise driven gear teeth needed to recalibrate your speedometer, ensuring accurate speed readings. Avoid false readings, potential tickets, and maintain your vehicle's correct mileage tracking with this essential tool.
Formula:
The formula used to determine the required speedometer driven gear teeth is:
Required Driven Gear Teeth = (Tire Diameter × Rear Axle Ratio × Drive Gear Teeth) ÷ Speedometer Cable Revolutions Per Mile
- Tire Diameter (TD): Overall diameter of your vehicle's tires in inches.
- Rear Axle Ratio (AR): Your vehicle's final drive ratio (e.g., 3.73:1).
- Drive Gear Teeth (DGT): The number of teeth on the drive gear located inside your transmission.
- Speedometer Cable Revolutions Per Mile (CPM): A vehicle-specific constant, often 1000 for GM and 1010 for Ford vehicles, indicating how many revolutions the speedometer cable makes per mile at 60 MPH.