What is the name of the law that says that Hummers and other REALLY HEAVY cars don't have to follow MPG rules?

Question

I am some research and I need to find out the specific law that states that Hummers and other heavy vechiles (Ford F-350s, Doge Ram 3500s and others) that don't have to follow the MPG rules. Is it a federal law? Can states change/alter it by passing their own law? If Hummers are exempt from the MPG, why don't they have to follow the same rules as 16-wheelers with the safety checks and stuff. PLEASE HELP!!! Also, I don't need websites of biased opinions, this is a factual essay/research project.

Any and ALL help is appreciated!!

Answer 1

I think there is such a law is not  in effect.

Answer 2

Its not a law. A law is a restriction. These things have no restrictions.