[/HRI've said it before and some may not agree but I have ran 235-85-16 Toyo M55's and am currently running Cooper Discoverer AT-3's. Neither are the best for mpg's but I do mileage calculations for every tank and noticed very little difference between the two.
Here's where I did notice a difference. The ride and drive. The 285's hold more weight @65psi than the 235's do @90psi. On highway or off, the bigger tires simply handle better all the way around and the truck rides noticably better. I wouldn't buy another set of 235's after running the two sizes and comparing the difference. You may see a 1/4mpg difference in favor of the smaller tire, not worth it for my purposes.