Yes, MILEAGES is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, L 1, E 1, A 1, G 2, E 1, S 1.

Is it a valid word?

MILEAGES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: fuel consumption rate, gasoline mileage, gas mileage.