gearshift vs mechanical device

gearshift and mechanical device both are nouns.

Word	Noun	Adjective	Verb	Adverb
gearshift	Yes	No	No	No
mechanical device	Yes	No	No	No

As nouns, mechanical device is a hypernym of gearshift; that is, mechanical device is a word with a broader meaning than gearshift:

gearshift: a mechanical device for engaging and disengaging gears
mechanical device: mechanism consisting of a device that works on mechanical principles

gearshift (noun)	mechanical device (noun)
a mechanical device for engaging and disengaging gears	mechanism consisting of a device that works on mechanical principles