How to Calculate Speedometer Gear Teeth
Calculating the correct number of teeth for a speedometer driven gear is crucial for maintaining accurate speed readings after changes to your vehicle's drivetrain or tire size. This calculator helps determine the appropriate number of teeth for the driven gear based on the drive gear, axle ratio, and tire diameter.
The formula used for calculating the number of teeth on the driven gear is:
\[Driven Gear Teeth = \frac{Drive Teeth \times Axle Ratio \times Rev/mile}{1,001}\]
Where:
- Driven Gear Teeth: The number of teeth on the speedometer driven gear
- Drive Teeth: The number of teeth on the drive gear
- Axle Ratio: The gear ratio of the differential
- Rev/mile: Revolutions per mile, calculated as 20,168 divided by tire diameter in inches
Calculation Steps
- Calculate Rev/mile: \[Rev/mile = \frac{20,168}{Tire Diameter (inches)}\]
- Determine the number of teeth on the drive gear.
- Identify the axle ratio of your vehicle's differential.
- Apply the formula: \[Driven Gear Teeth = \frac{Drive Teeth \times Axle Ratio \times Rev/mile}{1,001}\]
- Round the final number to the nearest whole number to get the recommended number of teeth for the driven gear.
Example Calculation
Let's calculate the number of driven gear teeth for the following scenario:
- Drive Gear Teeth: 6
- Axle Ratio: 3.73
- Tire Diameter: 29 inches
- Calculate Rev/mile:
\[Rev/mile = \frac{20,168}{29} = 695.45\]
- Apply the formula:
\[Driven Gear Teeth = \frac{6 \times 3.73 \times 695.45}{1,001}\]
- Calculate:
\[Driven Gear Teeth = \frac{15,557.58}{1,001} = 15.54\]
- Round to the nearest whole number:
\[Driven Gear Teeth = 16\]
Therefore, the recommended number of teeth for the driven gear is 16.
Speedometer Gear Diagram
This diagram visually represents the difference in the number of teeth between the drive gear and the calculated driven gear. The difference in teeth count allows the speedometer to accurately measure vehicle speed despite the gear reduction in the differential.