I personally don't see a vast different in my milage. I think that having your tires up at 35 will wear out he centers much faster. Just try driving around with the tires between 20-25 and check the tirewear and mileage. I honestly think that you will see a big improvement on ride quality and your mileage won't be affected. Your tire wear should also be better. I bet the 39 IROKs have a max pressure of 35. That would be if your loading it down with a bunch of stuff in your bed and towing that you'd want 35.
Like I said though just try it and tell me how it rides and how the tires wear.
you can always do the "chalk test" to see how they are gonna wear and how they sit
