gas mileage vs gasoline mileage

gas mileage and gasoline mileage both are nouns.

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

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

gas mileage and gasoline 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, mileage.

gas mileage (noun)	gasoline mileage (noun)
the ratio of the number of miles traveled to the number of gallons of gasoline burned	the ratio of the number of miles traveled to the number of gallons of gasoline burned