The upcoming Euro 7 regulation introduces the concept of continuous On-Board (emission) Monitoring (OBM), while On-Board Fuel/Energy Consumption Monitoring (OBFCM) is already an integral part of modern vehicles. The current work aims to assess whether on-board data could provide sufficient information to characterize real-world vehicle performance and emissions. Nine Euro 6d-ISC-FCM passenger cars were used, covering a wide range of powertrain technologies, from conventional gasoline and diesel to hybrid (HEV) and plug-in hybrid (PHEV) electric vehicles. Three vehicles were thoroughly tested in the laboratory and on the road, aiming at evaluating in detail the on-board data monitoring system. The evaluation concerned OBFCM device recordings of fuel consumed and distance travelled, as well as tailpipe NOx emissions and exhaust mass flow rate. Four vehicles were monitored for two weeks under real-world operation, while a diesel and a gasoline PHEV have been monitored for approximately two years. The collected data provided information on the evolution of fuel/energy consumption, highlighting the impact of parameters like season, mileage accumulation and charging behavior. The findings reveal good accuracy of the OBFCM device, with the lifetime fuel consumed deviation remaining below 5% in the Worldwide harmonized Light vehicles Test Cycle (WLTC) and slightly exceeding that threshold on the road in limited cases, while distance travelled deviations were negligible. Regarding NOx emissions, the tailpipe levels calculated using the on-board data were within ±10% compared to reference values, with the final deviation attributed to the combination of the inaccuracies of the sensor and the exhaust mass flow determination. Further findings of the mid/long-term monitoring activity highlighted the strong potential of on-board data to provide a realistic picture of vehicle operation under any driving conditions, e.g., evolution of the fuel/energy consumption and the real-world gap between laboratory and real-world values, characterization of the brake energy recuperation strategy of vehicles with different electrification levels and the identification of Diesel Particulate Filter (DPF) regeneration and of the corresponding NOx emissions.