he expressions

to switch gears, to shift gears
are often (too often for my taste, but that is a different matter) used to announce a switch from one topic to another in an oral presentation (e.g., a scientific talk).

As far as vehicles are concerned, gears are switched to change speed, not direction. Hence, I would rather associate a "switch of gears" with a change in the speed of the presentation. Therefore, expressions such as "to change lanes" or "to change direction" seem to be more appropriate.

Does anyone have knowledge about the origin of the phrase? I could not find it in the respectable dictionaries, so I'm also not sure how common it really is.

(Another interesting point is that the phrase seems to be particularly popular in the United States, despite the dominance of automatic cars.)
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This is one of those idioms that just is -- it doesn't really bear close examination, but everyone (who was raised in the US, at least) knows what it means. – Hot Licks Oct 17 '14 at 21:20
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Also shift gears with an equivalent meaning. – bib Oct 17 '14 at 23:54
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I'd like to stress that as a Briton, I am not familiar with this expression, so it's likely that @Hot Licks is right about it being a 'chiefly US' expression. – Pharap Oct 18 '14 at 8:58

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the dictionary.com definition uses the word "abrupt". Does that mean that the expression "Let's switch gears a little" (often used if the new topic is quite closely related) is contradictory? – painfulenglish Oct 18 '14 at 8:32
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@painfulenglish - Is there ever an idiom that isn't used in a contradictory manner at some point? – Hot Licks Oct 18 '14 at 11:51

In a close-ratio transmission, neighboring gears don't differ that much in size, so the transition from one gear to another wouldn't be too abrupt. So you could stretch the idiom to "switching gears a little." – Gnawme Oct 18 '14 at 17:25
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http://english.stackexchange.com/questions/203093/why-is-to-...