"In the 14 years since EPA established the Tier 2 Vehicle Program, manufacturers of light-duty vehicles and automotive technology suppliers have continued to develop a wide range of improved technologies capable of reducing vehicle emissions.... This technological progress has made it possible for manufacturers to achieve emission reductions well beyond the requirements of the Tier 2 program if gasoline sulfur levels are lowered further.
As a result, in conjunction with lower gasoline sulfur standards, we are establishing new Tier 3 standards for exhaust emissions "
"the program phases in over several years—with the primary vehicle emission standards starting in Model Year (MY) 2017 (2018 for heavier vehicles) and the gasoline sulfur control provisions beginning in 2017. "
I can't paste tables but emissions of NOX reduce to about 1/3 of the Tier 2 standards in 2025. How much of this is due to the low sulfur and how much is due to cars meeting the new CAFE requirements I don't know. I also don't know how close cars are now, from the read it definitely sounds like most are beating tier 2 even with more sulfur in the fuel.
