STL 9.1.4.3.C.3:
Either the OEM transmission or an alternate transmission must
be used; the alternate transmission must be from the same
manufacturer as the vehicle (e.g., an Acura transmission may be
installed in a Honda car). Alternate transmissions must be used in
their entirety. Retrofitting OEM complete gear sets in an alternate
transmission case is permitted.
I suggest the rule's intent is to allow swapping of OEM parts/transmissions/transaxles. I don't think it allows you to change gear ratios to something not OEM. I, for example, am using the Type R transaxle and gearset in my GS-R Integra.
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