Currently, light trucks are allowed to emit up to five and a half times more smog-forming pollution than cars. Beginning in 2007, a new regulation will mark the first time that SUVs and other light-duty trucks—even the largest passenger vehicles— will be subject to the same national pollution standards as cars.
The Federal Standards

The rule of thumb on the federal standards is this: the lower the bin number, the cleaner the vehicle. The Toyota Prius is a very clean Bin 3, while the Hummer H2 is a dirty Bin 11.

* Tier 2 bin 1: The cleanest Federal Tier 2 standard. A zero-emission vehicle (ZEV)
* Tier 2 bins 4 – 2: Cleaner than the average standard
* Tier 2 bin 5: "Average" of new Tier 2 standards, roughly equivalent to a LEVII vehicle