The tank size was taken from a long standing rule for class M bikes as a
minimum size to establish it as a sit up type bike with other appropriate
restrictions and misapplied to class A.
Class M was never meant to be a class that was restricted to former
street legal bikes but rather production based of the type that included
dirt and road race production bikes.
Look at the record book.
The misapplication of the maximum tank size for class A has no basis in
the history of the class and and has been demonstrated to be invalid
for a large fuel burner for example.
Russ stated here the rule originated with AMA before his time.
That was and is wrong with examples printed here and we haven't seen him since.