The present invention relates to a drive train of a wheeled vehicle having an engine and at least one driving wheel, the drive train essentially including a first clutch connected to the engine for receiving driving force therefrom and for outputting the driving force in a variable manner according to driver operation driver; a transmission connected to the first clutch effective for receiving the driving force from the engine as varied by the driving transmitting it a varying rotational speed; and a second clutch connected to the transmission for receiving such driving force from the transmission and transmitting it to the driving wheel by automatically varying the transfer ratio between the transmission and the driving wheel according to the rotational frequency of the transmission output. The method essentially includes the steps of; setting a transfer ratio at the first clutch means according to an operation of a driver; automatically setting a transmission ratio at the transmission; and automatically setting a transfer ratio at the second clutch means according to a rotational frequency of the driving force outputted by the transmission.