Gas Mileage

I was looking at the new smart car to come over from Europe to the usa. The car gets 60mpg in Europe but the first government tests show that with emission controls the car only get 37 mpg. Sounds weird and seems wrong but if you look at all the small econo box cars they all get around 35-37 mpg. Could this be government mandated mpg. If you look at the hybrids like the prius they use a dirty engine to get better mileage. If you took out the electric motor and batteries it would get the same mpg. They get around the emission rules with the electric motor. Also since vw tdi cars got killed off for 2007 and the 2008 don't get as much mileage, something is up. Could there be laws in place to make the oil companies more money. I remember reading about a car years ago that got less emissions without the catalytic converter and the government made them put the cat back in.