A vehicle's speedometer gives the driver an accurate estimation of the vehicle's speed. The speedometers on stock vehicles are calibrated for use with the vehicle's stock parts. Changing the stock parts, including the vehicle's tyres, can throw off the speedometer calibration; larger tyres cause the speedometer to read a speed slower than the vehicle's actual speed while smaller tyres cause the speedometer to read a speed faster than the vehicle's actual speed. You may recalibrate a speedometer in a few simple steps.
Open the vehicle's bonnet and locate the transmission. Unscrew the speedometer cable from the transmission's tail shaft and use an adjustable spanner to unscrew the bolt holding the gear housing cover in place. Remove the plate to expose the two gears: the drive gear mounted inside the transmission and the driven gear attached to the speedometer cable. Count the number of teeth on the drive gear and note the number.
Measure from the top of one of the vehicle's wheels to the centre of the wheel. Multiply this number by two to determine the diameter of the tyre. Divide 12,532 by the tyre's diameter to determine the tyre's revolutions per km and divide 20,168 by the tyre's diameter to determine the tyre's revolutions per mile.
Multiply the vehicle's revolutions per km by the amount of drive gear teeth determined in step 1. Multiply this value by the vehicle's axle ratio. Look for the vehicle's axle ratio in the supporting documents that came with your vehicle. It may also be found inside the vehicle's glove box. Divide the solution by 1,001. The resulting number is the number of driven gear teeth needed.
Contact the vehicle's manufacturer or a local speedometer service location and purchase the necessary driven gear for your vehicle. You must provide the make, model and year of your vehicle to ensure you receive the right gear.
Remove the old driven gear from the end of the speedometer cable by pulling it out of the clips that hold it in place. Clip the new driven gear into place on the speedometer cable. Replace the gear housing cover and secure it in place with the bolt you removed in step 1. Secure the speedometer cable back into position.
Determine the distance of the test drive needed to recalibrate your speedometer. This information may be in your owner's manual or other supporting documentation, but it is also available from the vehicle's manufacturer.
Press and hold the calibration button on the speedometer, start the vehicle and release the button.
Press the button again and drive the distance prescribed by the speedometer manufacturer.
Press the button a third time after you have driven the designated distance. After pressing the button the third and final time, the speedometer will recalibrate itself to fit the new tyre size.