This paper investigates energy efficiency of a series hybrid electric vehicles (HEV) that utilizes a continuously variable transmission (CVT) to connect the electric motor to the wheels. In contrast with a fixed transmission FT that employs a fixed final drive ratio, the CVT offers variable transmission ratio that can be freely controlled, so that the motor is driven more efficiently. The performance of the CVT is evaluated within an optimal control framework under an urban drive mission, which is specified in terms of the road geometry and the traveling time for the journey. Apart from the CVT operation, vehicle speed and the energy management are also simultaneously optimized by an indirect optimal control method, based on the Pontryagin's minimum principle (PMP). The simulation results illustrate the benefit of the CVT as compared to a fixed transmission in terms of fuel economy.