gasoline mileage vs gas mileage

gasoline mileage and gas mileage both are nouns.

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

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

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

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

gasoline mileage (noun)	gas 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