The shaft of the car engine transfers power to the gearbox, and the transmission ratio from the output shaft of the gearbox to the wheels is constant. A "soft shaft" is mounted on the output shaft of the gearbox, which goes straight into the mileage meter in front of the driver. The so-called "soft axis" is like the bicycle line brake with the pull wire with steel wire core of the spiral tube, the wall and the inner core between the lubricating oil, the outer tube is fixed and the inner core can rotate, the inner core speed and the wheel speed has a constant proportional relationship. The soft axle connects to the speedometer so that the pointer can indicate the speed of the car. At the same time, by soft shaft rotation of worm and worm wheel to speed table in the middle of the roller on the counter, turn the wheels of the number represented by the mileage accumulated down, because of the speed and distance are all on the same root of flexible shaft rotation driven, so these two tables together, the former use pointer instructions, the latter by roller counter cumulative.