yokahama tires

Im Wyatt and im new to the site. I have a few questions involving tire sizes and the decrease in mpg itll cause. Bone stock 1998 K1500 4x4 with 5.7L and a 3.73 rear end. Came stock with 245/75/16 and ive already stepped up a size to 265/75/16 and saw a slight decrease in gas mileage. Obviously...