US Federal law is fair:

- No cars are exempt from emission control laws. It is against the law to remove or defeat emissions controls (no matter what the age)

- Older cars are not required to retro-fit newer controls, or to upgrade to newer limits. For example, a 1970 model only has to meet the 1970 requirements

- There is no federal emissions test

- When importing a non-US model into the US, an emissions waiver is automatically granted if the car is more than 25 years old (or somewhere thereabouts)

Each State has their own rules. Some states have no periodic vehicle tests, other do. For example:

- usually only in urban areas

- usually only for a limited time. In California, 1974 and older are exempt from testing. In Washington, 1978 and older are exempt from testing

- Washington's test is "every two years"

- Washington doesn't open the hood and visualy inspect the systems. California performs a visual inspection

- dyno testing or static test (idle + 35mph)

- Washington recently moved to testing the fuel cap -- does it hold the correct pressure and relieve excessive pressure?