Until 2017 in Europe the Type Approval (TA) procedure for light duty vehicles for the determination of pollutant emissions and fuel consumption was based on the New European Driving Cycle (NEDC), a test cycle performed on a chassis dynamometer. However several studies highlighted significant discrepancies in terms of CO2 emissions between the TA test and the real world, due to the limited representativeness of the test procedure. Therefore, the European authorities decided to introduce a new, up-to date, test procedure capable to closer represent real world driving conditions, called Worldwide Harmonized Light Vehicles Test Procedure (WLTP). This work aims to analyze the effects of the new WLTP on vehicle CO2 emissions through both experimental and simulation investigations on two different Euro 5 vehicles, a petrol and a diesel car, representatives of average European passenger cars. The study also considers the effect of the engine warm-up and the impact of the start-stop technology in this new TA scenario. Since the WLTP imposes higher test mass and Road Loads (RLs), as well as higher driving cycle dynamics, a 44% cycle energy demand increase for the petrol car and a 23% increase for the diesel car were found. However, CO2 emissions increased in the same proportion only for the diesel car, while they increased only by 10% for the petrol car, thanks to the improvement of the average internal combustion engine efficiency along the WLTC cycle. Finally, the effectiveness of the start-stop in terms of fuel (or CO2) savings, was found to be almost halved for both vehicles when passing from the NEDC to the WLTP.