You're correct when you say that the difference isn't in diesel vs gasoline. But for better or for worse, US regulations (read: California Air Resource Board (C

You're correct when you say that the difference isn't in diesel vs gasoline. But for better or for worse, US regulations (read: California Air Resource Board (CARB) regulations, not federal/EPA regulations) are much more stringent in terms of particulates and NOx, while EU laws focus more on CO2 emissions. California is primarily worried about smog in LA, while Europe is generally much more concerned with maintaining the North Atlantic Current. As you said, the high efficiency gasoline engines generate more NOx and particulates, but these engines cannot pass a CARB smog test. Most cars have separate US vs EU specs, (or simply aren't available in one market) and the EU specs always have better power and gas milage. For instance, the 1.0L engine in my 2014 Ford Fiesta is rated for 123hp, but the same 1.0L engine in the 2014 EU spec Ford Fiesta gets 140. (EPA vs EU gas milage ratings aren't directly comparable) This is a direct result of the fact that the US version runs at a lower temperature, which necessarily results in lower power and efficiency, but also results in less NOx. Incidentally, this is a large part of why the US response to the VW emissions scandal was so different than the EU response. The affected VW engines broke EU regulations by a little, but broke CARB regulations by a lot. It's like going 70mph in a 55mph zone vs a 35mph zone.