mileage vs gasoline mileage

mileage and gasoline mileage both are nouns.

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

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

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

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

mileage (noun)	gasoline mileage (noun)
distance measured in miles	the ratio of the number of miles traveled to the number of gallons of gasoline burned
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