Speedometer Gear Planning Guide
A mechanical speedometer depends on a matched drive gear and driven gear. The drive gear sits on the transmission output shaft. The driven gear turns the cable or sensor. Tire diameter, axle ratio, and gear teeth decide the cable speed. When one value changes, the gauge may read high or low.
Why This Calculator Helps
Transmission swaps often change the drive gear count. Taller tires also reduce tire revolutions per mile. A deeper axle ratio increases output speed for the same road speed. This calculator combines those factors. It recommends a driven gear tooth count and shows the nearest practical option. It also estimates the indicated speed error for an installed gear.
Key Input Choices
You may enter a direct tire diameter. You may also build diameter from width, aspect ratio, and rim size. The direct value is useful for measured tires. The tire size method is useful during planning. Enter the axle ratio as a decimal, such as 3.73. Enter the drive gear teeth inside the transmission. Use the target cable revolutions per mile from your gauge or adapter. Most classic mechanical speedometers use one thousand revolutions per mile.
Reading The Results
The exact driven gear is often not a whole number. Real gears are sold with whole tooth counts. The rounded recommendation gives the closest standard choice. The comparison table checks each candidate. A positive error means the gauge reads faster than actual speed. A negative error means the gauge reads slower than actual speed. Pick the tooth count with the smallest absolute error.
Practical Notes
Always confirm available gears for your transmission family. Housing angle, gear pitch, and shaft style can vary. Tire diameter also changes under load. Measure rolling diameter when accuracy matters. Drive a measured mile after installation. Then compare the odometer or speed reading. Use the correction factor to decide whether a different gear is needed. The calculator gives a strong starting point. Final road testing confirms the best setup for your vehicle.
Keep records for every setup you test. Save the CSV before buying parts. The file makes changes easier to review. The PDF gives a workshop build sheet. Keep it for future repairs after installation work too.