How big are the factory gas tanks in '73-'74 A bodies?

I have a '74 Duster with a mild 440. I am getting about 10mpg (:glasses7:) and am wondering since my gas gauge doesn't work... how big are the factory gas tanks? I am thinking it is 16 gallons but... I am not sure.

I have been using my GPS to keep track of my speed (speedometer doesn't work) and my mileage. If I knew how big of gas tank I have, I could figure out how far I can go between fill ups.

Does any one know?