my E350 has 245/75/16's now and I am wondering if I switch to the 265/75/16's what I could expect in mpg
You could just as easily lose mileage. There really isn't any good way of predicting what your mileage will be.
With tire size, there's always a sweet spot. Go any smaller than the sweet spot, and you lose mileage. Go any larger, and you lose mileage. Here's just one quick and dirty way of looking at it:
Some of it has to do with your driving style and what your vehicle primarily does most of the time. For instance, those who mostly drive highway might see a slight gain with larger tires because you're running lower average RPM's, but will suffer mileage loss when driving around town. In contrast, if you strictly drive around town (constant start/stop), then you would probably gain with smaller tires, since there is less rolling resistance to overcome when accelerating from a stop.
In short, just because you're going to a taller tire, doesn't mean you'll gain mileage. The only way to know for sure is to try it. If you lose mileage, then go back down in size.
Of course, there are plenty of other factors too, such as wind resistance (increases with vehicle height), tread design (highway tires get better mileage than all-terrains/mud tires), current gearing, etc.