First, determine how many gallons are in the Ford F150's fuel tank. In most cases, the Ford F150 fuel tank will hold about 19 gallons of gasoline or diesel. To find out how many miles the Ford F150 has remaining, divide the total gallons of fuel by the number of gallons in the tank. So if a Ford F150 has a fuel tank that holds 19 gallons of gasoline and it has 3,000 miles left on its odometer, then the Ford F150 would have approximately 2,700 miles' worth of gas left in its tank.