This is on a 4 litre?
Nothing to speak of for mileage results.
Power you will see a small gain at wide open throttle due to the programmer.
Basically its going to sound better, and pass it bit better on the high way. Otherwise there wont be much change. The main restriction in 4 litres is the cylinder head, and valve timing due to emission control. Until thats addressed, its very difficult for the 4 litre to take any advantage at all from things like intakes and exhausts.
The chip on the other hand advances ignition timing and increases injector pulse at WOT. Thats where you see the gains.
Unless its an offroad specific program, at which point they are allowed to change the tuning throughout the entire rpm range. The WOT thing is due to emission standards and regulations, etc etc.
Hope that helps