Speedometer Error Calculator
Actual Speed:
mph
km/h
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How to Calculate Speedometer Error
When changing the size of tires on a vehicle, it's important to consider the accuracy of the speedometer. The speedometer measures the number of axle revolutions to determine the vehicle's speed. Different tire sizes can affect the accuracy of the speedometer readings, as larger wheels rotate less and smaller wheels rotate more to cover the same distance. Fortunately, you can calculate the speedometer error and make corrections when adjusting tire sizes.
To estimate the speedometer error, you need to know the diameter of the new and old tires. Follow these steps to calculate the actual speed of the vehicle:
1. Find the ratio of the new tire diameter to the old one.
2. Multiply the ratio by the speedometer reading to estimate the actual speed of the vehicle.
The formula to calculate the actual speed of the vehicle with a new tire size is:actual speed = (new diameter / old diameter) × speedometer(alert-success)
By dividing the new tire diameter by the old diameter and multiplying it by the speedometer reading, you can estimate the actual speed of the vehicle. You can also use a tire comparison calculator to determine the new and old tire diameters.
How to Correct Speedometer Error After Changing Tire Size
Once you identify speedometer error after installing new tires, it's necessary to correct it. The process for recalibrating the speedometer depends on whether it's electronic or mechanical.
1. Recalibrating an Electronic Speedometer:
- Consult the vehicle's documentation to find the process for resetting the speedometer and the prescribed driving distance.
- Press and hold the speedometer calibration button, start the vehicle, and release the button.
- Press the button again, drive the prescribed distance, and press the button once more to recalibrate the speedometer to the new tire size.
2. Recalibrating a Mechanical Speedometer:
- Look at the vehicle documentation to determine the axle ratio.
- Determine the overall diameter of the new tire using a tire size calculator or by measuring it. Divide the diameter by 20,168 to find the revolutions per mile.
- Locate the speedometer cable attached to the transmission's tail shaft. Unscrew the cable and remove the gear housing cover to expose the gears.
- Count the number of teeth on the drive gear.
- Calculate the number of teeth needed on the new driven gear using the formula:
driven gear teeth = (rev/mile × drive gear teeth × axle ratio) / 1,001
- Find a new driven gear with the correct number of teeth and replace the existing gear.
- Reinstall the housing cover, reattach the speedometer cable to the transmission, and ensure everything is properly secured.
By following these steps, you can calculate and correct speedometer error after changing the tire size on a vehicle.
