The conveyor speed to carry vehicle bodies into assembly lines to complete the assembly process is very important for managing and increasing the daily production and productivity. In this paper, using discrete simulation technique and a simulation methodology, which is designed by simulation technical committee of Michigan University, we modeled and simulated the mechanical assembly line of RIO vehicle in SAIPA Corporation to find the optimum speed of conveyor, such that the daily production and total productivity are increased as reasonable and feasible. The speed of conveyor in mechanical assembly line of RIO vehicle is 0.9 m/s. Also, the daily output is in arrival [53, 69]. After various running of simulated system of mechanical assembly line of RIO, we found the optimum status of conveyor speed such that the outputs and productivity are increased. In this regard, if the conveyor speed in mechanical assembly line of RIO vehicle is increased as much as 0.6 m/s, the daily output of mechanical assembly line will be in arrival [57, 73], where both, the lower and the upper bounds of the assembly line production will increase as much as 4 units or vehicle bodies.