In this article, the gear ratio of RD (An Iranian made car by Iran Khodro Co.) vehicle gearbox with considering fuel economy and gearbox specifications is evaluated. In the first step, the gearbox advantages and its effects on the engine rotational velocity with considering road load and engine torque are investigated. It is distinguished that in a specified velocity of vehicle, engine speed in overdrive state is very lower than engine speed in fourth gear. It means that noise and fuel consumption and engine wearing and damages will be decreased. The optimized region of engine operation is identified. Using a geometric progression between automotive gear ratios and entering number of effective parameters such as specific fuel consumption, minimum mean effective velocity, and etc., overdrive gear ratio is computed. Finally the overdrive gear ratio is chosen 0.81 for vehicle.

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