I recently upgraded from the stock Continentals that came on my truck which were approx. 33X10.5X18 to the Toyo Open Country M/T in a larger 35X12.5X18 tire (same rolling diameter as you are looking at.
I am geared with the stock 3.73 gears.
To make a long story short, with the stock tires when I purchased the vehicle I was averaging ~15MPG, after several service appointments and all the software upgrades, I had recently been averaging ~13MPG... a bit disappointing. After switching to the new Toyo tires, my calculations show that I am getting a dismal 9.8 MPG. However........
I have not had the computer re calibrated for the 35" tires, so my speed, and overall mileage is most likely off by a bit, therefore my MPG calculations are also off by a bit i'm sure.
As far as performance goes, I have to say i'm pretty pleased. The larger, M/T tread does make a little noise..... sort of like a constant whir when traveling down the road. It also takes a little more time to get everything up to speed, but the off-road performance, and performance in rain and especially snow is fantastic. For my purposes I'm pretty happy.