I lost 1-2 mpg going from 245/75/16 Michelin LTX A/S to 265/70/16 BFG A/T. I'd expect to gain most, if not all, of the mpg back by going to stock size (245/75/16) Michelin A/T. I'm sure I'd gain some back just by going to stock size BFG A/T's. Stock size Michelin A/T's would be my pick for best mpg with good off road traction. Rotate/balance every 5,000 miles & they'd probably last 80,000+ miles. When Michelin was rating them, they rated 9 out of 10 for mud traction. You can read reviews at TireRack.com.
I gained a little mpg with each of the following: adding a topper; 100% synthetic oil in motor, transmission, differentials; Michelin tires (low rolling resistance); driving technique.
05 Tacoma, V6, 4x4