Vehicle Thermal Management System (VTMS) is a crosscutting technology affecting the fuel consumption, engine performance and emissions. With the new approved fuel economy targets and the enhanced vehicle performance requirements, the ability to predict the impact on the fuel consumption of different VTMS modifications is becoming an important issue in the pre-prototype phase of vehicle development.This paper presents a methodology using different simulation tools to model the entire VTMS in order to understand and quantify its behavior. The detailed model contains: engine cooling system, lubrication system, powertrain system, HVAC system and intake and exhaust system. A detail model of the power absorbed by the accessory components operating in VTMS such as pumps and condenser is presented. The power of the accessory components is not constant but changing with respect to engine operation. This absorbed power is taken into account within the power produced by the engine shaft. The results obtained by using the integrated model are in a good correlation with the measurements from vehicle tests.The integrated model is used to study the thermal transient behavior of the vehicle under a given driving cycle (e.g. NEDC). The energy flow within the vehicle and the amount of it consumed by each component during the driving cycle are observed and evaluated with this model.The sensitivity of the key parameters affecting the fuel consumption and vehicle performance is studied. Based on the sensitivity analysis, the best known modifications of each key parameter are integrated together in order to evaluate the global fuel consumption reduction potential.