The majority of published data on exhaust gas emissions is cited based on the test results from the NEDC driving cycle. This paper specifies the methodology of how to develop dynamic characteristics of the emissions, based on the aggregated data from the measurement of modal emissions obtained in the course of NEDC and FTP-75 driving cycles for gasoline and CNG fuelling. The presented characteristics concern a real passenger car which was tested, then the characteristics were used to calculate the exhaust gas emissions during a specially developed driving test comprising four main motion resistances and engine braking. The effect of fuel type on selected emissions of exhaust gas components has also been compared and assessed.