How to Calibrate an Odometer

Test the accuracy of the odometer. For a simple but crude testing method, carefully roll to a stop at a highway mile marker. Write down the odometer's reading. Drive along the shoulder until reaching the next mile marker. Write down the odometer's reading. This can only be accurate to within a tenth of a mile. Use calibration machinery for higher accuracy.

Calculate the odometer's ratio. Many American cars are set to a 1,000 to 1 ratio. This means that for every 1,000 spins of the output shaft, the odometer dial moves one tenth of a mile. A gear ratio is determined by adding the number of teeth on two gears and dividing the first gear by the second gear. For example, if gear one has 100 teeth and gear two has 20 teeth, the gear ratio is 5:1.

Start calibrating the odometer by disassembling it. A bicycle's odometer is simply a little wheel placed against the wheel. In a car, the odometer spins within a metal housing. The mechanism of a mechanical odometer is a series of worm gears that turn the subsequent gear one tooth at a time. The final gear turns the one tenth of a mile indicator which in turn moves the one mile, ten mile and so on.

Adjust the odometer's gear ratio. This can be done by interrupting the spinning of the output shaft connected to the transmission. Use fine needle nose pliers to crimp the output shaft as it enters the odometer. This will restrict the spinning of the shaft.

Calibrate a computerized odometer using calibration instruments. This requires an investment of time and money. This equipment is not readily available to the public as it can be used to re-set a used car's odometer and thus defraud a buyer. The diagnostics in a car dealership can easily pick up tampering.