**Additional EPA Follow**
**- Up to March 12, 2013 Phone Call on Tier 3** 

**Edits in response to questions from 3/12 about LD GHG and criteria pollutant impacts**  Excerpt from Preamble Section III.B. As discussed throughout this preamble, implementation of the proposed Tier 3 standards is aligned with the 2017 LD GHG standards to achieve significant criteria pollutant and GHG emissions reductions while providing regulatory certainty and compliance efficiency to the auto and oil industries. The 2017 LD GHG stand ards were still in a preliminary state of development (pre

- proposal) at the time we finalized our assumptions for the Tier 3 emissions, air quality , and cost anal ys es, so we were not able to reflect them in these anal ys es . However , w e continue to expect v ehicle criteria pollutant performance to be neutral under the GHG program , because exhaust and evaporative emissions are not proportional to the amount of fuel burned; rather, our standards are expressed on a per- mile basis, not on a per

- gallon basis. Veh icle criteria emissions are almost exclusively

controlled by

a vehicle’s emissions aftertreatment sy stem and not by the efficiency of the engine. The majority

of the NMOG that is emitted from a gasoline engine is generated during cold start, before the cataly st is lit off, and NOx is often created during higher load operation. Optimizing cataly st efficiency , minimizing thermal parasitics, minimizing fuel sy stem leaks , and lower gasoline sulfur will be key enablers for all vehicles to meet the Tier 3 st andards, regardless of the vehicle’s fuel efficiency

. Because we do not expect the increase in fuel efficiency to result in lower criteria pollutant emissions, we did not claim in the 2017 LD GHG rule any reductions attributable to the ability

of vehicles to meet lower criteria emission levels (s ee 77 FR 62899

- 62901, October 15, 2012)

. In other words, in the 2017 LD GHG rule, we assumed that, a bsent the proposed Tier 3 standards , the light

- duty fleet would continue to meet the Tier 2 standards. Thus , we believe that the inclusion of the light

- duty GHG standards in our cost and benefit anal ys es for the proposed Tier 3 standards would have had little or no impact on the results or our conclusions, as discussed in Sections 7.1.2 and 7.1.3.2.1 of the draft RIA. Nevertheless , for the final rulemaking we will include the LD GHG requirements in the analy sis.**Edits in response to questions on GHG benefits/disbenefits from Tier 3 vehicle standards**

Excerpt from Preamble Section IV.A.5 Since there are multiple aspects to the Tier 3 program, it is necessary to consider technical feasibility

in light of the different program requirements and their interactions with each other. In many cases, manufacturers would be able to address more than one requirement with the same general technological approach (e.g., faster catal ys t light

- off can improve both FTP NMOG+NO X and PM emissions). At the same time, the feasibility assessment must consider that different technologies may be needed on different types of vehicle applications (i.e., cars versus trucks) and must consider the relative effectiveness of these technologies in reducing emissions for the full useful life of the vehicle while operating on expected in

- use fuel. For example, certain smaller vehicles with correspondingl y small engines may be less challenged to meet FTP standards than larger vehicles with larger engines. Conversel y, these smaller vehicles may

have more difficulty

meeting the more aggressive SFTP requirements than vehicles with larger and more powerful engines. Additionally , the ability to meet the proposed SFTP emission requirements can also be impacted b y the path taken to meet the FTP requirements (e.g., larger volume cataly sts for US06 emissions control vs. smaller catalysts for improved FTP cold

- start emiss ions control). Throughout the following discussion, we address how these factors, individually

and in interaction with each other, affect the feasibility of the proposed program. We invite comment on our assessments of these or any other such potential i nteractions. Also, although we are not aware of any technological reasons that vehicle emission controls responding to new Tier 3 requirements should affect vehicle CO 2 emissions or fuel econom y in any significant way, it is possible that such interaction s could occur. For example, there may be some slight reduction of vehicle mass if manufacturers explore lighter exhaust manifold materials in order to reduce thermal mass and promote earlier cataly st light

- off. W e invite comment on any

such potential eff

ects as well.