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 standards 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 analyses, so we were not able to reflect them in these analyses. However, we continue to 

expect vehicle 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. Vehicle criteria emissions 

are almost exclusively controlled by a vehicle’s emissions aftertreatment system 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 catalyst is lit off, and NOx is often created during higher load operation.

Optimizing catalyst efficiency, minimizing thermal parasitics, minimizing fuel system leaks, and 

lower gasoline sulfur will be key enablers for all vehicles to meet the Tier 3 standards, 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 (see 77 FR 62899-

62901, October 15, 2012). In other words, in the 2017 LD GHG rule, we assumed that, absent 

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 

analyses 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 analysis.
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 catalyst light-off can improve both 

FTP NMOG+NOX 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 correspondingly small engines may be less challenged 

to meet FTP standards than larger vehicles with larger engines. Conversely, 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 by the path taken to meet the FTP requirements 

(e.g., larger volume catalysts for US06 emissions control vs. smaller catalysts for improved FTP 

cold-start emissions 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 

interactions. Also, although we are not aware of any technological reasons that vehicle emission 

controls responding to new Tier 3 requirements should affect vehicle CO2 emissions or fuel 

economy in any significant way, it is possible that such interactions 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 catalyst light-off. We 

invite comment on any such potential effects as well.