Do the adjectives “reliable” and “dependable” have the same exact meaning?
If not, what is the difference and when is best to use each of them?
The OED gives the following definitions of reliable and dependable:
Reliable—1. That may be relied on.
1a. Of a person, information, etc.: able to be trusted; in which reliance or confidence may be placed; trustworthy, safe, sure.
1b. orig. U.S. Of a product, service, etc.: consistently good in quality or performance; dependable.
Dependable—That may be depended on; trustworthy, reliable.
Both definitions list the other word in their definitions, with no special qualifiers attached. In addition, both words use the word trustworthy, also without qualifiers. Therefore, it is safe to say that the two are synonyms and may be equally used.
Here is the discussion of reliable and dependable in Webster's Dictionary of Synonyms (1942):
A person or thing is reliable when one can count on him or it not to fail in doing what he or it is expected to do competently (as, she is a very reliable servant; one of the most reliable of our employees; a reliable washing machine), or to give or tell the exact truth (as, a reliable work of reference; reliable testimony. A person or thing is dependable to whom (or which) one can go in full confidence that one will get the support or assistance required in time of need or in an emergency; as, to ask a friend to recommend a dependable physician; he is the most dependable of our friends; a dependable source of information. Dependable is also used merely as a descriptive term implying a character that admits nothing that is incalculable, or that is the antithesis of that which is fickle, capricious, or the like. "Laura wasn't pretty, but ... healthy-looking and dependable" (M. Austin).
Here is the corresponding discussion in James Fernald, Funk & Wagnalls Handbook of of Synonyms, Antonyms & Prepositions, revised edition (1947):
Reliable denotes the possession of such qualities as are needed for safe reliance; as a reliable pledge, reliable information. A man is said to be reliable with reference not only to moral qualities, but to judgment, knowledge, habit, or perhaps pecuniary ability. ... A dependable person or thing can be relied on to help whenever a need arises[.]
Interestingly, the first edition of Fernald's dictionary (1896) opens with the comment that "The word reliable has been sharply challenged, but seems to have established its place in the language." Fernald then goes on to show why the criticism "on the ground that the suffix -able can not properly be added to an intransitive verb" is baseless, and then compares reliable to trustworthy and trusty—but not to dependable, which it doesn't cover at all.
Finally, here is the discussion of the two terms in S.I. Hayakawa, Choose the Right Word (1968):
Reliable suggests competence and consistency: A reliable judge is one who has a record of sound opinions. A reliable person can be counted on to do what he has promised or been told to do: a reliable baby sitter.When applied to things, reliable means adequate, serviceable, or true; a reference book, for example, might be called reliable if the information it presents is accurate. Dependable is akin to reliable, but is a little more subjective; reliable is often used of relationships based on service between superiors and inferiors, whereas dependable more often suggests an attitude of personal allegiance rather than one of honesty or scrupulosity in the performance of a duty. One goes to a dependable person confident of receiving loyalty, support, or aid: a dependable ally. When applied to things, dependable suggests stability and consistency of performance: a dependable drug.
As with all books that attempt to delineate fine distinctions between words that have similar meanings, the ones I cite here inhabit a gray zone where distinguishing between actual differences in preponderant informed usage (on the one hand) and the authors' desire to establish differences that may not be widely observed in practice (on the other) can be difficult.
Obviously "reliable" means "can be relied upon" and "dependable" means "can be depended upon", but both my Webster's and etymological dictionary give near-identical usages, even using the other as synonyms.
Google Ngram viewer shows "reliable" as far more common than "dependable".
Reliable and dependable are interchangeable when they refer to things (i.e. a clock), or to oneself vis-a-vis someone else. However, there is a nuance when the terms apply to someone else vis-a-vis oneself. In this case reliable implies a decision to commit oneself to another and to acept the consequences in the event of failure. With dependable, the commitment is not a free choice. (Source: The Heritage Illustrated Dictionary of the English Language, International Edition, McGraw Hill.)