40 CFR 91.204 - Averaging.
(a) Negative credits from engine families with FELs above the applicable emission standard must be offset by positive credits from engine families below the applicable emission standard, as allowed under the provisions of this subpart. Averaging of credits in this manner is used to determine compliance under § 91.207(b).
(b) For model years through 2000, outboard credits may not be summed with personal watercraft credits, or vice versa, for purposes of compliance under § 91.207, except manufacturers may, at their discretion, include personal watercraft credits with outboard credits upon demonstration to the satisfaction of the Administrator that the personal watercraft engine is installed in a hybrid vessel that is smaller than a typical sterndrive or inboard vessel and larger than a typical personal watercraft. For model year 2001 and later, manufacturers must sum credits generated from outboard and personal watercraft to determine compliance under § 91.207.
(c) Credits used in averaging may be obtained from credits generated by another engine family as allowed under § 91.204(b), in the same model year, credits banked in the three previous model years, or credits obtained through trading.