As predicted..
http://abcnews.go.com/Business/ceo-c...ry?id=14131519
The U.S. EPA has a hard on for both oxides and particulates when it comes to diesels vs. Euro EPA has a hard on for fuel economy and CO2 emissions. In order for diesels in the country to meet US EPA regulations, they HAVE to fun slightly lower A/F ratios to reduce the NOx output and subsequently add in a particulate filter to catch the particulate matter. Then, diesel fuel needs to be injected post compression in order to heat the converter up so these particulates can be burned. Both of these equate to lower fuel economy.
The Europeans on the other hand allow for higher oxides and higher PM, but require lower fuel consumption and lower CO2 output. In order to lower CO2 output, you can really only lower HC/CO output by reducing fuel usage.