I've read a number of posts about modifying the gearing when going to a larger tire size. When is it necessary? Is the loss of power and mpg significant enough to require modified gearing when only increasing to 265/75/16 BFG KO's? I currently have a 4.636 ratio. Thanks.

No

No but you may need a flux capacitor

Does that flux capacitor run on 1.21 gigawatts?

No regearing needed.

But be sure to drop by the tire section and ask if those 265/75s will rub.

Thanks for the info.