49 C.F.R. § 536.4
Link to an amendment published at 85 FR 25275, Apr. 30, 2020.
(c) Adjustment factor. When traded or transferred and used, fuel economy credits are adjusted to ensure fuel oil savings is preserved. For traded credits, the user (or buyer) must multiply the calculated adjustment factor by the number of its shortfall credits it plans to offset in order to determine the number of equivalent credits to acquire from the earner (or seller). For transferred credits, the user of credits must multiply the calculated adjustment factor by the number of its shortfall credits it plans to offset in order to determine the number of equivalent credits to transfer from the compliance category holding the available credits. The adjustment factor is calculated according to the following formula:

Where: A = Adjustment factor applied to traded and transferred credits. VMTe = Lifetime vehicle miles traveled as provided in the following table for the model year and compliance category in which the credit was earned; VMTu = Lifetime vehicle miles traveled as provided in the following table for the model year and compliance category in which the credit is used for compliance; Model year Lifetime vehicle miles traveled (VMT) 2011 2012 2013 2014 2015 2016 2017-2025 Passenger Cars150,922177,238177,366178,652180,497182,134195,264 Light Trucks172,552208,471208,537209,974212,040213,954225,865 MPGse = Required fuel economy standard for the originating (earning) manufacturer, compliance category, and model year in which the credit was earned; MPGae = Actual fuel economy for the originating manufacturer, compliance category, and model year in which the credit was earned; MPGsu = Required fuel economy standard for the user (buying) manufacturer, compliance category, and model year in which the credit is used for compliance; and MPGau = Actual fuel economy for the user manufacturer, compliance category, and model year in which the credit is used for compliance.
[74 FR 14452, Mar. 30, 2009, as amended at 75 FR 25727, May 7, 2010; 77 FR 64051, Oct. 18, 2012]