Vin 1FTSW2BR3AEA90750. What is my gear ratio?

According to the VIN you have a limited slip 3.55 rear diff.

https://researchmaniacs.com/VIN-Number-Lookup/WindowSticker/FordSticker.html? title=1FTSW2BR3AEA90750

Thanks

Any idea what type of mpg I can expect?

A diesel F250? Probably 10 to 17. Less if you tow or haul. How about a best answer?

Thanks, is there a turner, and or other mods. You’d recommend?

I do not recommend tuners especially for the fragile 6.4.

As far as mods you should look into the bullet proof mods before you blow the motor up. https://bulletproofdiesel.com/collections/6-4l-ford-parts