ruby-encoding-map

ruby-encoding-map is a variable defined in `ruby-mode.el'.
Its value is

((us-ascii)
 (shift-jis . cp932)
 (shift_jis . cp932)
 (japanese-cp932 . cp932))

• This variable may be risky if used as a file-local variable.

Documentation:
Alist to map encoding name from Emacs to Ruby.
Associating an encoding name with nil means it needs not be
explicitly declared in magic comment.

You can customize this variable.