EU standards reflect the reality that diesel engines are far more common than in the US, so they are less strict with regards to NOx emissions to begin with. Ab

EU standards reflect the reality that diesel engines are far more common than in the US, so they are less strict with regards to NOx emissions to begin with. About half the km (or miles, or furlong) driven in Europe are on diesel. Diesel gets better mileage and tends to be cheaper than gas (depending on local taxes), which can make a significant difference with the high fuel prices in Europe. The oversight is also much more fragmented because it is less centralized than in the US. As I understand it, the oversight is left to the member states, so manufacturers tend to shop around for the most lenient one.