Calculate the odometer ratio. Many American cars are set to a 1,000 to 1 ratio. This means that for every 1000 turns of the output shaft, the odometer dial moves one tenth of a mile. The 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.