49 C.F.R. § 536.4
(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 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 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. The quotient shall be rounded to 4 decimal places;
[74 FR 14452, Mar. 30, 2009, as amended at 75 FR 25727, May 7, 2010; 77 FR 64051, Oct. 18, 2012; 85 FR 25275, Apr. 30, 2020]