meter reading vs mph

meter reading and mph both are nouns.

Word	Noun	Adjective	Verb	Adverb
meter reading	Yes	No	No	No
mph	Yes	No	No	No

As nouns, mph is a hyponym of meter reading; that is, mph is a word with a more specific, narrower meaning than meter reading:

meter reading: a datum about some physical state that is presented to a user by a meter or similar instrument
mph: a speedometer reading for the momentary rate of travel

Other hyponyms of meter reading include clock time, time, miles per hour.

meter reading (noun)	mph (noun)
the act of measuring with meters or similar instruments	a speedometer reading for the momentary rate of travel
a datum about some physical state that is presented to a user by a meter or similar instrument	the ratio of the distance traveled (in miles) to the time spent traveling (in hours)