Ford typically allows 10% reserve after empty or 1.8 gallons for a 18 gallon tank. Add to that the number of miles to empty and you can get an idea of how much fuel is left in the tank. If you're averaging 20 mpg and you're 50 miles to empty that's 2.5 gallons. 2.5 + 1.8 = 4.3 gallons remaining.