A speedometer includes a function storage device, which has stored therein four conversion functions, a selector, which provides four options corresponding to the four conversion functions in the function storage device, and a microprocessor, which counts pulse signals sent by the sensor device of the motor vehicle to obtain a counted total value, selects the corresponding conversion function from the function storage device corresponding to the option selected through the selector according to the quantity of poles (two, four, eight or sixteen) on the drive shaft of the motor vehicle, and puts the counted total value into the selected conversion function for calculation to obtain a car speed value.