ruby-insert-encoding-magic-comment

ruby-insert-encoding-magic-comment is a variable defined in `ruby-mode.el'.
Its value is

t

Documentation:
Insert a magic Ruby encoding comment upon save if this is non-nil.
The encoding will be auto-detected. The format of the encoding comment
is customizable via `ruby-encoding-magic-comment-style'.

When set to `always-utf8' an utf-8 comment will always be added,
even if it's not required.

You can customize this variable.