MPG is affected by the width and height of your tires, as well as by the tread. The loss of MPG is caused by a bigger tire patch, which results in increased mechanical drag, and by the bigger profile which results in increased aerodynamic drag. Wheel size doesn't matter - it's the meat you put on those wheels that determines any gain or loss in MPG.
How much you loose depends on where you're coming from. In your case you had stock LT235/85R16E tires. Those are 9.3" wide and 31.7 diameter.
The stock tires on Ford 20" rims are LT275/65R20. Those are 11 inches wide and 34" diameter. So almost two inches wider and over 2 inches taller. If you stick with that size tire I'd guess you'll lose about two to three MPG - assuming a calibrated speedo both before and after the change in tire size.
The biggest reasonable-size tire you'd want to run on those 20"x8" rims is 285/65R20. Those are 11.5" wide and 34.7" diameter - i.e., the same as a lot of so-called 35s. Anybody with 35s - and that knows how to calculate accurate MPG - will tell you they cost them about 3 MPG - or more with a more-agressive tread .