NOx emissions from diesel cars in the EU have remained high, largely due to a growing gap between emission certification limits of Euro 4 and Euro 5 standards (measured in laboratory testing) and “real-world” emissions of diesel cars operating on the roads. Beginning in 2017, the EU’s emissions type-approval procedure for passenger cars will include a new real-driving emissions (RDE) test conducted using on-board portable emissions measurement systems (PEMS). This paper examines the potential impact of the RDE regulation and ICCT’s proposed modifications on real-world emission factors of new diesel cars and projected passenger car fleet NOx emissions in the EU through 2030. Findings indicate that real-world NOx emissions from new diesel cars in the EU could be reduced from 5–7 times the Euro 6 limit to 4 times that limit with the introduction of the RDE test. Future changes to the RDE test and European type-approval and enforcement practices recommended by the ICCT could further reduce new diesel car NOx emissions to 1.2 times the Euro 6 limit.