mileage vs gas mileage

mileage and gas mileage both are nouns.

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

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

mileage and gas 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, gasoline mileage.

mileage (noun)	gas 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