You can't estimate your actual fuel MPG by using your fuel gauge. The fuel gauge is an approximation based on the vehicle sitting on level ground. Because the fuel level sender is near the rear of the tank if you park on a steep hill (nose-down) the fuel gauge can easily show a 1/4 tank less than in would if the vehicle is sitting on level ground. Conversely, if you park on a steep hill nose-up the fuel gauge can show 1/4 tank more than it would on level ground. The only way to know what your actual fuel economy is is to fill your tank, set your trip odometer to zero, drive a distance (lets say 100 miles), refill your tank and do the math (number of miles driven divided by number of gallons used = your MPG). 6 gals used in 100 miles traveled = 16.66 miles per gallon.
Your fuel economy is based on may variables. Traffic, speed, terrain, tire pressure, load, your vehicles gear ratio, engine state of tune, mechanical condition, even the wind. My V6 2WD '01 Spor Trac with 4:10 gears gets around 17 MPG average, but this can vary to be between 14-20 MPG depending on conditions.
Again, what is your actual MPG? The Explorer Sport Trac is not a very fuel efficient vehicle. If you're expecting to see more that 17 'ish MPG average, you probably will not. Frankly my V8 Explorers typically get better fuel economy than my V6 ST. Oh, and 4x4's get worse mileage than the 2WD's because the 4x4's are heavier.
You stating that you put in $20 and that gave you 1/2 tank is meaningless w/out knowing how much fuel was in the tank before you bought gas and how much per gallon gas cost when you bought the gas.
