Thank you for the extra insight, I may have had the wrong conclusion but I think the logic still follows. From this system I outlined, the manufacturers would be forced to alter designs to get the desired emissions during normal driving conditions. This is the whole spirit of the regulation right?

My point is that the system you outline doesn’t work, because ordinary drivers can’t check their own emissions. It would be fine for fuel consumption since that’s easily measured, but emissions requires special equipment and careful testing.

NOx emissions specifically are a problem because the engines are run very efficiently at high burning temperatures with complete combustion.