Remembered why I got a F450 with 4.30 gears ... observation.

The fuel savings will obviously depends on the mileage you put every year.
As I wrote above, the difference in mpg on my trucks was huge.
Both had the same engine, both were flatbeds, one was dually, other 4WD what makes them about equal in rolling resistance.
So the only big factor for over 30 percent different mpg was gearing.