Offroad-Express

Hi I'm hoping that someone could help me with their experiences or theories on weather if I changed my tire size from a 31x10.5x15 to a 33x12.5x15 would it improve or worsen my fuel economy. I drive 90 percent highway 10 percent around town. It's a bj74 cruiser with a 13bt an auto with standard diff ratios. Thanks for any help.

Worse, the bigger tyre will be heavier and have more wind drag so will take more power to turn. I doubt it would be more than 1l per 100k difference though.