A simple version of a Series-Parallel Hybrid Electric Vehicle (SPHEV) powertrain was proposed and studied in previous work. It consists of 2 Electric Machines (EM) mounted on the engine shaft and separated by a clutch. It is a relatively simple architecture that allows vehicle operation in pure electric, series hybrid and parallel hybrid modes. However, this architecture was proven to be less efficient than a reference Power-Split SPHEV architecture: the Toyota Hybrid System (THS). As a continuation, this paper investigates the potential of improvement of the simple SPHEV by the addition of gears and a gearbox.Two new variants of SPHEVs are therefore proposed. The three versions of SPHEVs and the reference THS are sized on a global optimal manner and compared. Results show that one variant of the simple SPHEV can have comparable fuel consumption to the THS.