The shaft of an automobile engine transfers power to the transmission, and the gear ratio from the output shaft of the transmission to the wheels remains constant. A "flexible shaft" is mounted on the output shaft of the transmission, extending all the way to the odometer in front of the driver. A "flexible shaft" is a spiral tube with a steel core like the cable used in bicycle brakes, with lubricant between the tube wall and the core. The outer tube is fixed while the core can rotate, and the rotation of this core is in a constant proportion to the wheel's rotation. The flexible shaft connects to the speedometer, allowing the pointer to indicate the vehicle's speed. Additionally, the rotation of the flexible shaft is also transmitted to the speedometer's roller counter through worm gears, accumulating the mileage represented by the wheel rotations. Since both speed and mileage are driven by the same rotating motion of the flexible shaft, these two gauges work together: the former indicated by the pointer, and the latter counted by the roller counter.