The proposed Tier 3 standards will have a certification "bin" (Bin 160 - one of seven), which effectively equates to the current Tier 2 Bin 5. However, the vehicle fleet must
average Bin 30 (i.e., the equivalent of Tier 2 Bin 2), so there probably will be very few vehicles that are certified to Bin 160 once the Tier 3 regulations are implemented.
The change for diesels from Tier 2 is that NMOG and NOx are now combined into a single standard (like Europe has been) which give the manufacturers of diesel cars more flexibility in meeting that standard (NOx is no longer distinctly regulated).
Here is a comparison of the proposed Tier 3 Bin 160 (the highest certification bin under Tier 3) and Euro 6...
(grams/mile)
Emission.................
Tier 3 Bin 160...........................................
Euro 6
NMOG+NOx................0.160....................................................0.274
CO...........................4.2.......................................................0.8
PM...........................0.003...................................................0.007
There's also a particle number (PN) Euro 6 standard of 6X10^11 particles/km (~2X10^12 particles/mile) which is probably much more restrictive than the PM mass standard (0.007 g/mi). EPA is not proposing a PN limit for Tier 3. There's also a NOx limit for diesels in Euro 6 (0.08 g/km = ~0.13 g/mi), but none for Tier 3.
The proposed Tier 3 regs also include supplemental FTP (SFTP) standards which regulate emissions in more aggressive test duty cycles (US06 and SC03); Euro 6 does not have supplemental standards.
More information can be found at
http://www.dieselnet.com/standards/us/ld_t3.php#bins and
http://www.dieselnet.com/standards/eu/ld.php#stds.