A transmission device of a vehicle is equipped with a shaft, a driven wheel with internal gear teeth defined thereon mounted to the shaft center thereof, and a small driving gear with outer gear teeth defined thereon in mesh with the internal gear teeth of the driven wheel wherein, depending on the position of the shaft, the small driving gear is located at the offset center of the driven wheel therein to serve as a main source of power supply. When the small driving gear, carrying the total weight and load of the vehicle, is rotated forwards and upwards at the driven wheel therein in eccentric manner, the outer gear teeth of the small driving gear tend to climb upwards therewith along the internal gear teeth of the driven wheel thereof to rotate the driven wheel therewith, effectively converting the gravity force into dynamic driving power to reduce the output of power requirement for more efficient and economical use thereof. Besides, a limiting device with damping springs is disposed at one side of the driven wheel to keep the small driving gear located at one half of the driven wheel in eccentric gearing and maintain the output of gravity force in transmission.