Good info. I figured the 235/85/16s would fit since nearly all Toyota trucks/suvs can take upto a 32in tire with no prob, I didn't think the 255/85/16s would fit though, interesting food for thought on my part.
Another factor with your Coopers are the fact they are a wider tire, but I'd figure the extra height would help offset the mpg loss.
Based on
Tire Size Calculator
235/85/16 = 31.7in tall, 9.5in wide
255/85/16 = 33.1in tall, 10.04in wide
Looking at your tank fillups, it looks like the offroad tires pushed you down to around 23.5 and with the street tires you got 25.5ish as of lately, so around a 7-8% loss in mpg.
I suspect going from 10.5in wide to 9.5in wide should help some to off set some of the mpg loss for the tires I have in mind.