Regulatory compliance measurements for vehicle emissions are generally performed in well equipped test facilities using chassis dynamometers that simulate on-road conditions. There is also a requirement for obtaining accurate information from vehicles as they operate on the road.An on-board system has been developed to measure real-time mass emission of NOx, fuel consumption, road load, and engine output. The system consists of a dedicated data recorder and a variety of sensors that measure air-to-fuel ratios, NOx concentrations, intake air flow rates, and ambient temperature, pressure and humidity. The system can be placed on the passenger seat and operate without external power.This paper describes in detail the configuration and signal processing techniques used by the on-board measurement system. The authors explain the methods and algorithms used to obtain (1) real-time mass emission of NOx, (2) real-time fuel consumption, (3) road load, and (4) engine output. Correlations with laboratory regulatory compliance tests show discrepancies within 4% for NOx mass emission measurements, within 3% for fuel consumption measurements and within 1% for running distance measurement.Using the system, the authors observed that the distance-based NOx mass emission measurement is negatively correlated with fuel economy. The study confirms a tendency for NOx reduction as ambient humidity increases and reports on observations of changes in fuel consumption with seasonal changes in the humidity and density of ambient air.