I can't imagine it NOT being able to change your speedo calibration. As far as fuel mileage, that is dependent on the current efficiency of the vehicle. A good tune will never make it worse. The truck will feel lighter and MPG gains will be minimal, maybe 1 sometimes 2 mpg on the average vehicle. Thats a good tune in general, I never run canned tunes and I'm not personally familiar with Diablo/Diablew except for the mostly good things that has been reported.
If you were closer I would help you out, I don't do this for monetary gains, its just a hobby.