[xxpor]

>I've always thought that this was because NA has air pollution laws that are strictly and honestly enforced and that that would make diesel cars difficult to offer.

Yes, the US regulates NOx emissions much more than Europe. This makes it very hard to offer passenger diesel engines in the US. On the other hand, the EU regulates/taxes CO2 emissions, which the US does only indirectly through CAFE (fuel efficiency) regulations.

[oblio]

How is US fuel efficiency, though?

In Europe cars using 5 liters per 100 km (~48 miles per gallon for the SI-resistant amongst us) are very common.

[xxpor]

Much worse than that. There may be a few non-hybrid models that can get that high, but they're very uncommon. You have to remember the best selling "car" in the US is a Ford F-150. They're also exempt from CAFE as I understand it, as they're classified as light trucks rather than cars.

This almost certainly has more to do with how low our gas tax is compared to almost every other developed country though, than any direct regulation.

[fomine3]

CO2 emission is almost equal to fuel efficiency. So the regulation explain why.