mileage vs travel allowance

mileage and travel allowance both are nouns.

Word	Noun	Adjective	Verb	Adverb
mileage	Yes	No	No	No
travel allowance	Yes	No	No	No

As nouns, travel allowance is a hypernym of mileage; that is, travel allowance is a word with a broader meaning than mileage:

Other hypernyms of mileage include travel reimbursement.

mileage (noun)	travel allowance (noun)
distance measured in miles	a sum allowed for travel
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