Have you been resetting the onboard at each fill-up? Have you seen a decrease in the average MPG's via the onboard? Has hand calculating shown a decrease also? The on-board is inaccurate but useful with respect to trending.
Since a cars average fuel mileage can vary greatly from tank-to-tank, it's normal for the miles per tank value to change. A drastic change in miles per tank will also be related to a drastic change in average fuel mileage. How many miles on the car? When was the last tune-up? Any brake issues? Tire pressures ok? You've given us nothing to go on.