gas mileage vs mileage

gas mileage and mileage both are nouns.

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

As nouns, gas mileage and mileage are synonyms defined as:

gas mileage and mileage: the ratio of the number of miles traveled to the number of gallons of gasoline burned

Other synonyms of gas mileage include fuel consumption rate, gasoline mileage.

gas mileage (noun)	mileage (noun)
the ratio of the number of miles traveled to the number of gallons of gasoline burned	distance measured in miles
	a travel allowance at a given rate per mile traveled
	the ratio of the number of miles traveled to the number of gallons of gasoline burned