The paper presents the main results of a study on the simulation of energy efficient management of on-board electric and thermal systems for a medium-size passenger vehicle featuring a parallel-hybrid diesel powertrain with a high-voltage belt alternator starter. A set of advanced technologies has been considered on the basis of very aggressive fuel economy targets: base-engine downsizing and friction reduction, combustion optimization, active thermal management, enhanced aftertreatment and downspeeding. Mild-hybridization has also been added with the goal of supporting the downsized/downspeeded engine performance, performing energy recuperation during coasting phases and enabling smooth stop/start and acceleration.The simulation has implemented a dynamic response to the required velocity and manual gear shift profiles in order to reproduce real-driver behavior and has actuated an automatic power split between the Internal Combustion Engine (ICE) and the Electric Machine (EM). Typical parallel hybrid technology functions, such as Stop&Start, regenerative braking and power assistance from the EM have all been implemented in the GT-Drive model.After model calibration and validation versus the available experimental data, the energy management strategies of such a hybrid configuration were investigated. The results obtained for the New European Driving Cycle (NEDC) and a Real Life Driving Cycle (RLDC) have been discussed, in terms of fuel economy and performance.