speedbar-tag-hierarchy-method

speedbar-tag-hierarchy-method is a variable defined in `speedbar.el'.
Its value is

(speedbar-prefix-group-tag-hierarchy speedbar-trim-words-tag-hierarchy)

Documentation:
List of hooks which speedbar will use to organize tags into groups.
Groups are defined as expandable meta-tags. Imenu supports
such things in some languages, such as separating variables from
functions. Each hook takes one argument LST, and may destructively
create a new list of the same form. LST is a list of elements of the
form:
(ELT1 ELT2 ... ELTn)
where each ELT is of the form
(TAG-NAME-STRING . NUMBER-OR-MARKER)
or
(GROUP-NAME-STRING ELT1 ELT2... ELTn)

You can customize this variable.