gas mileage vs ratio

gas mileage and ratio both are nouns.

Word	Noun	Adjective	Verb	Adverb
gas mileage	Yes	No	No	No
ratio	Yes	No	No	No

As nouns, ratio is a hypernym of gas mileage; that is, ratio is a word with a broader meaning than gas mileage:

gas mileage: the ratio of the number of miles traveled to the number of gallons of gasoline burned
ratio: the relative magnitudes of two quantities (usually expressed as a quotient)

gas mileage (noun)	ratio (noun)
the ratio of the number of miles traveled to the number of gallons of gasoline burned	the relation between things (or parts of things) with respect to their comparative quantity, magnitude, or degree
	the relative magnitudes of two quantities (usually expressed as a quotient)