You have to put in more than 10 Gallons of Gas to get the system to reset and reestimate the miles to empty status. Since you have a full tank, either you have some bad circuit or sensor (call a dealer), or I would run it completely down near empty, and then put in 10 or more gallons and see if it resets. Perhaps it has gotten confused.
20 miles to the gallon
The Mitsubishi Montero Sport offers 16 miles to the gallon in the city. The Montero Sport also offers 21 miles to the gallon on the highway.
I have a 1999 Mitsubishi Montero Sport XLS with 270,000 and still consistently average 18-20 mpg
The EPA mileage estimate for the 2001 Mitsubishi Mirage is 27 mpg in the city, 35 mpg on the highway.
The EPA mileage estimate for the 2004 Mitsubishi Lancer is 24 mpg in the city, 32 mpg on the highway.
The EPA mileage estimate for the 2003 Mitsubishi Lancer is 23 mpg in the city, 30 mpg on the highway.
The EPA mileage estimate for the 2012 Mitsubishi Lancer is 25 mpg in the city, 34 mpg on the highway.
The EPA mileage estimate for the 2002 Mitsubishi Lancer is 23 mpg in the city, 30 mpg on the highway.
The EPA mileage estimate for the 2006 Mitsubishi Outlander is 19 mpg in the city, 25 mpg on the highway.
The EPA mileage estimate for the 2004 Mitsubishi Outlander is 19 mpg in the city, 24 mpg on the highway.
The EPA mileage estimate for the 2005 Mitsubishi Outlander is 19 mpg in the city, 25 mpg on the highway.
The EPA mileage estimate for the 2010 Mitsubishi Lancer is 22 mpg in the city, 31 mpg on the highway.