If your cooling fan is engaged constantly, there is a good chance the engine is not warming up to normal operating temperature. If the engine does not reach temperature, you will not be getting the best fuel mileage you could be getting because the fuel injection will be operating as if the engine is cold. The Explorer seems to have a standard fan clutch. When the engine is off, and cold, you can normally feel how much resistance the fan has when you turn it. If the clutch is locked up, the fan will not turn or be very difficult to turn. Does the fan seem to be louder than usual? This is not an electric fan right?