PURPOSE: To enable the display of normal speed and running range on a running regardless of the diameter of a tire to be replaces, by providing a gear for detection of rotation detachably on a speed meter mounted on end of a rotating shaft at the final stage of a transmission gear through a wire.

CONSTITUTION: Power changed with a main transmission gear 5m and a subtransmission gear 5s is transmitted to a rotating shaft 16 on the downstream side thereof and then to a rear wheel via a coupling 19 and a propelling shaft 20 from a middle shaft 18 through bevel gears 17 and 17. A rotation detector section 8 is provided on the end of a shaft 16 estended from a through hole 21a of a case 21 to detect the rotation proportional to the rotation of a rear wheel from the rotation of the shaft 16. A pair of meshed gears 23 and 24 for detection of rotation are housed in a cover 22 while the cover 22 is mounted detachably on the outside of the case 21. Thus, the gears 23 and 24 as housed in the cover 22 can be separated with the removal of the cover 22 from the case 21. Even when a tire is replaced by the one of a different diameter, the gears 23 and 24 are exchanged for those with the transmission ratio corresponding to the diameter of the tire thereby enabling the inputting of normal rotation into a speed meter.