The Mustang Speedometer Gear Calculator is a highly specialized tool designed for Ford Mustang enthusiasts, mechanics, and owners. This calculator helps users accurately determine the right speedometer gear setup for their vehicle, ensuring the speedometer reads correctly after modifications such as tire size changes or differential gear swaps. By inputting specific information about their vehicle’s tire size, rear axle gear ratio, and transmission type, users receive precise calculations for the necessary speedometer gear teeth. This tool is essential for maintaining the accuracy of the vehicle’s speed reading, which is crucial for both legal compliance and the vehicle’s performance.
Formula of Mustang Speedometer Gear Calculator
Calculate Tire Diameter
Tire Diameter (in inches) = (Tire Width * (Tire Series / 100) * 2) / 25.4 + Rim Diameter
- Tire Width is the tire width in millimeters.
- Tire Series is the aspect ratio, a percentage that describes the tire sidewall height.
- Rim Diameter is the diameter of the rim in inches.
Calculate Tire Revolutions Per Mile (RPM)
Tire Revolutions Per Mile = 63360 / (Tire Diameter * π)
- 63360 is the number of inches in a mile.
- π (Pi) is approximately 3.14159.
Determine the Drive Gear Teeth Based on Transmission Type and Selection
This step requires selecting between:
- 6 Tooth
- 7 Tooth
- 8 Tooth
based on the transmission description you provided.
Input Rear End Axle Gear Ratio
Input directly based on your choice, e.g., 3.55:1, 4.10:1, etc.
Final Speedometer Gear Calculation
Gear Teeth = (Drive Gear Teeth * Axle Ratio * Tire Revolutions per Mile) / 1001
- Drive Gear Teeth is determined in step 3.
- Axle Ratio is the rear end axle gear ratio.
- Tire Revolutions per Mile is calculated in step 2.
General Terms and Conversion Table
| Term | Description |
|---|---|
| Tire Width | The width of the tire measured in millimeters (mm). |
| Tire Series | The aspect ratio of the tire’s sidewall height to its width in percentage (%). |
| Rim Diameter | The diameter of the wheel’s rim measured in inches (in). |
| Drive Gear Teeth | The number of teeth on the drive gear, which can vary based on the transmission type. Typically 6, 7, or 8 teeth. |
| Axle Ratio | The ratio of the rear differential, indicating how many times the drive shaft rotates for each turn of the wheels. |
| Tire Revolutions Per Mile (RPM) | The number of times the tire rotates in one mile of travel. |
This table serves as a foundational guide for understanding the terms used in the calculations.
Example of Mustang Speedometer Gear Calculator
For a Mustang with 255mm wide tires, a 40% series tire, 17-inch rims, a 6-tooth drive gear, and a 3.55:1 axle ratio, the calculations are as follows:
- Tire Diameter: Approximately 25.48 inches.
- Tire Revolutions Per Mile (RPM): About 798.79.
- Speedometer Gear Teeth Needed: Approximately 17.02.
To ensure accurate speedometer readings, a gear close to 17 teeth is recommended, adjusting to the nearest available size for practical application.
Most Common FAQs
When you change your tire size, you must recalculate the tire diameter and revolutions per mile to determine if a new speedometer gear is needed for accurate speed readings.
The axle ratio directly influences the calculation of the needed speedometer gear teeth, as it alters how many times the wheels rotate for each turn of the speedometer gear.
While this calculator is specifically designed for Ford Mustangs, the principles apply to any vehicle needing accurate speedometer calibration after modifications. However, the specific drive gear options may vary.
