This paper presents a study of passenger cars in terms of emissions measurements in tests conducted under real driving conditions (RDE - Real Driving Emissions) by means of PEMS (Portable Emission Measurement System) equipment. A special feature of the RDE tests presented in this paper is that they were performed under Polish conditions and the specified parameters may differ from those in most other European Union countries. Emission correction coefficients have been defined, based on the test results, equal to the increase (or decrease) of driving emissions during the laboratory (‘chassis dyno’) test or during normal usage in relation to the EU emission standards (emission class) of the vehicle. In relation to the fact that at present emissions limits specified for chassis dynamometer testing are currently used for RDE testing, in this paper we present those limits, proposing that they be accompanied by conformity range coefficients defining the “safety factor” for exhaust emissions measured during chassis dyno tests. On the basis of the coefficients defined in this paper, the ecological parameters of vehicles of various emission classes and design parameters (fuel supply systems) are presented. The test results and defined emission correction coefficients permitted an ecological assessment of vehicles of various emission classes to be conducted, and in the meantime they help with pre-emptive actions in terms of reducing emissions of some pollutants from passenger vehicles. The measurement methods applied here can also be used for other types of vehicles (e.g. heavy duty, off-road), or alternative fuel vehicles.