rack and pinion vs geared wheel

rack and pinion and geared wheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
rack and pinion	Yes	No	No	No
geared wheel	Yes	No	No	No

As nouns, geared wheel is a hypernym of rack and pinion; that is, geared wheel is a word with a broader meaning than rack and pinion:

rack and pinion: a wheel gear (the pinion) meshes with a toothed rack; converts rotary to reciprocating motion (and vice versa)
geared wheel: a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion

Other hypernyms of rack and pinion include cogwheel, gear, gear wheel.

rack and pinion (noun)	geared wheel (noun)
a wheel gear (the pinion) meshes with a toothed rack; converts rotary to reciprocating motion (and vice versa)	a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion