Ive been watching my mpg very closely too. The tank size is 17 gallons, but the range only gives you the range for 15 gallons. When my car said the range was 1 mile i put 15 gallons to fill it up, so that might be where your calculations arent adding up.
Im still only getting 15 mpg in the city and 23 on the highway, seems much lower than what the sticker said on the car. The dealer says that everything looks fine and the engine isnt running rich. Im not sure where the gas is going. Does anyone know what could be going on or is this something i just have to accept?