Portable Emissions Measurement Systems (PEMS) represent a robust and accurate solution to study the in-use emissions of combustion engines. The application of PEMS is now ranging from large engines to the smallest light-duty vehicles. The current commercially available PEMS exhibit measurement performances that are close to the ones of laboratory grade systems; when PEMS data are analysed with an adequate method, the test results allow a detailed insight into the on-road emissions performance of the vehicles with respect to their behaviour on the standard laboratory test cycles. The development of representative test cycles, which has become a typical approach in the last years, supposed to address special driving situations, now becomes less efficient because of the effort needed for their development and the poor representativeness of the results. In this light, PEMS testing offers an easy and efficient way to evaluate the vehicle emissions over a huge variety of conditions.This paper presents the on-road gaseous emission measurements of two light-duty diesel vehicles (Fiat Scudo 2 litre and VW T5, 2.5 litre) tested simultaneously with PEMS on the same routes including urban, extra-urban (rural) and highway during chase experiments. The vehicle absolute emissions and fuel economy is reported. The realworld emission results are analysed with an averaging window approach based on CO2 mass emission; this to provide a first qualitative insight on the variability of exhaust emissions from compression ignition combustion engines under real-world conditions. The investigation and methodology applied show much higher emissions values for real world data compared to the standard laboratory test cycles. The comparison of vehicle emissions and fuel economy performance allows an insight into the variability of emission behaviour under constant boundary conditions.