They are not stricter, just different.

US emissions standards don’t allow some things that euro6+ does, and visa versa.

There is an effort to make them overlap to make it easier to meet both standards.

How so? There is no way any of those engines would pass emissions in the EU. How are US regulations even stricter?

Back in 2015 US EPA NOx emission limits were tighter than EU regulations.

https://en.wikipedia.org/wiki/Volkswagen_emissions_scandal#U...

I feel this also has a lot to do with the use of diesel. The number of passenger vehicles and light duty pickups in the US that actually use diesel is a fraction of what it was in Europe.

The emission limits being for ULEV vehicles, I don't think I have seen a ULEV truck - indeed California classifies most under the LEV banner.

None of this is to change the point around nitrous oxide emissions - the environment doesn't care whether it came from a VW TDI or a F350, just the amount.

But it is also far far easier to implement such low emission standards in the US because we just don't really use diesel like that.

And when you get to the heavy duty pickups (F250, F350, etc.), then most of that goes out of the window.

The standard are similar but testing is different. In the US every new engine has to be tested, in the EU only the first R&D engines need to be tested on standard automated test cycles. All subsequent engines not. This makes cheating trivial and manifactoring cheap.