Just to add, 15.9 is the actual tank size, so lets imagine for a moment that you fillup with say, 1 gallon in the tank.

250 miles= 16.7 mpg
275 miles=18.4 mpg

and with say, 3 gallons left 19.3 mpg (@250)
and at 275 miles 21.3 mpg.

...sorry, i like numbers.

the most accurate way to tell us of an issue like that would to fill up your tank with x amount of gallons. when you reach a certain point, record mileage. divide and get an accurate mpg.