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This notice of proposed rulemaking includes corrections, clarifications, additional flexibilities, and adjustment factors to improve the Greenhouse gas Emissions Model (GEM) compliance tool for heavy-duty vehicles while more closely matching the outputs produced by the original GEM version 3.0 that was used to establish the CO2 standards for Model Years 2021 and later in the 2016 Heavy-duty Phase 2 final rule. This notice supplements the proposed rule published on May 12, 2020 (85 FR 28153), which included a larger set of proposed revisions to modify and improve GEM. Most of the proposed revisions from that notice of proposed rulemaking are addressed in a final rulemaking published in the Federal Register, titled "Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and other Technical Amendments," docket number EPA-HQ-OAR-2019-0307. Given the nature of this proposal, there will be neither significant environmental impacts nor significant economic impacts for any sector.