Since hydraulic hybrid vehicles are power dense and do not require costly batteries, they have the potential to achieve high fuel economy and performance and at low cost. The three main classes of hydraulic hybrid architectures are series, parallel and hydro-mechanical (HMT) or power-split. These architectures have intrinsic differences in transmission efficiencies and effectiveness in engine management. This paper compares the fuel economies and performance of these architectures and validates these features. Using a Toyota Prius like engine and chassis as common factors, fuel economies are compared for the 'optimal' design for each architecture which considers both the physical designs and the engine/energy management. Physical design variables include pump/motor sizes and gear ratios. The effect of pump/motors efficiencies, extra gears and different engine efficiency maps are also studied. To improve computational efficiency in evaluating fuel economy, engine operation is restricted to several operating modes and the accumulator pressure is assumed to be constant. These simplifications enable the Lagrange multiplier method to be employed so as to quickly determine the optimal engine management control and the resulting fuel economy for each design iteration. Full optimal control computations without the simplifying assumptions for the optimized design for each architecture (using dynamic programming) verify that, despite these simplification, the estimated fuel economies are close. It is shown that hybrid HMT offers the best fuel economy for various hydraulic efficiencies, followed by parallel and series architectures. However, the difference between HMT and parallel architectures diminishes if the engine has a wide efficient speed range of operation. It is also shown that an extra mechanical gear ratio can significantly increase fuel economy for all three architectures.