Abstract: In the last decade many researches have been carried out on wind driven vehicle, a large number of academic publications have been presented. There have been many Down Wind Faster Than The Wind arguments based on energy flow. Wind driven vehicle systems travel faster than the wind along its direction. This paper deals with energy conversion mechanism of a vehicle driven by wind. The Downwind vehicle is designed with a propeller that pushes it along like an aircraft propeller does. The propeller is connected to the wheels of the vehicle through a chain/belt drive, so that when the vehicle moves forward, the propeller spins in such a way to provide a thrust that will speed up the vehicle if there is energy available from a tailwind, which explains the energy conversion mechanism. Specifically, focus is on the horizontal axis propeller of a downwind vehicle to make it have a relative motion with the wind at a speed greater than the speed of the wind. The simulations of the analysis are carried out in JavaProp software.