I don't think this has been posted before. So here is the link. ICCT NOx White Paper Note the table on Page 2. It appears that the BMWs imported into the USA market (and presumably the Canadian Market as well) are using a more expensive, more complex and presumably better NOx emissions control system than anyone else anywhere in the world (including European BMWs). Unfortunately, the test data used in the study only includes the European configuration, so whether or not the North American diesels are actually cleaner in real world driving remains an unanswered empirical question, but it seems likely they are significantly cleaner (if calibrated correctly ).