hit it square on the head.
The US's stupid epa requirements are more concerned about emissions per fixed period. Because of that, diesel engines and their particulate emissions dont pass the spec, even though they are usually cleaner and less smog producing then the gas powered vehicles that do pass.
Now, if the government did something intelligent like emissions content per gallon of fuel consumed, then maybe we could have all the nice fuel efficient vehicles with small diesel engines that the rest of the world enjoys.
Shoot. I'd love 29 mpg at 3.01 a gallon, compared to my 16 mpg at 3:39 a gallon.