Speedometer Gear Calculator
Calculate the correct speedometer gear for accurate speed readings
Mechanical speedometers use a system of gears to convert wheel rotation into speed readings:
- The speedometer gear connects to the transmission output shaft
- Gear teeth count determines the ratio of wheel rotation to speedometer reading
- Different gear ratios are needed for different tire sizes and axle ratios
- Accurate readings depend on having the correct gear ratio
Common reasons for needing a new speedometer gear:
- Changing tire sizes
- Modifying the drivetrain
- Replacing worn gears
- Correcting inaccurate readings
Follow these steps to calculate the correct gear size:
- Measure your actual speed using a GPS or reference vehicle
- Note the speed shown on your speedometer at the same time
- Count the teeth on your current speedometer gear
- Enter these values into the calculator
- The calculator will show the required number of teeth for accurate readings
Common problems that affect speedometer accuracy:
- Tire Size Changes: Larger or smaller tires affect the actual distance traveled per revolution
- Gear Wear: Worn teeth can cause erratic or inaccurate readings
- Drivetrain Modifications: Changes to the transmission or differential affect the speed ratio
- Incorrect Gear Installation: Using the wrong gear size leads to consistent errors
Signs you need a new speedometer gear:
- Consistent speed reading errors
- Erratic speedometer behavior
- Speedometer needle fluctuation
- Noise from the speedometer cable
How accurate should my speedometer be?
Most vehicle manufacturers aim for speedometer accuracy within ±2.5%. Legal requirements vary by region, but generally allow for up to 10% error on the high side (showing faster than actual speed).
Can I install the gear myself?
While it's possible to replace the speedometer gear yourself, it requires some mechanical knowledge and proper tools. If you're not confident, have a professional mechanic do the installation.
Why does tire size affect speedometer accuracy?
Larger tires cover more distance per revolution than smaller tires, causing the speedometer to read slower than actual speed. Smaller tires have the opposite effect.