Stock tires on some models is 33", so an extra 2" isn't much of a difference. The 8 speed is smart enough to not purposely lug the engine, or gear hunt in/out of top gear at lower speeds, even 55mph. Take the new 200 sedan or Jeep Cherokee with the 9 speed auto, you need to keep a steady 80mph+ before it will go into 9th gear. EPA rates it on the highway in 8th gear as their highway test is around 60mph. Without enough torque to maintain momentum at any given speed, the transmission won't reluctantly upshift to cause additional wear on the drive train by having the engine lug. The transmission relies on engine rpm for cooling as well. Vehicles are smart.
As for the mileage loss, it's not just the tires alone, it's the lift. Increased drag at speed will lower mileage. The engine will also have to work harder, especially if you have 3.55 gears. Going with larger tires, and to not decrease performance the 3.92 gears is adequate, although if you plan to do any off roading it's best to go deeper, with 4.56 gears, and you'll find you'll get into top gear sooner, but I would never force a trans (manually shifting) to get into a higher gear that it doesn't want to, as the on board computer system does try to maximize proper cooling, and efficiency at any speed. Hence why manufactures spend multimillions on automatic transmissions, and a manual isn't even offered anymore.
With my stock 2011 Hemi Quad Cab 4x4 with 31.6" tires and 3.55 gears, I can cruse at 65mph at ~1645rpm in top gear which is comfortable for the weight of the vehicle. If I added a lift and 35" tires I would run ~1485rpm also with 3.55 gears. 3.92 gears would bump that up to ~1640rpm, roughly what my Ram does stock. Going deeper, with 4.56 gears, would up that more to roughly 1900rpm, limiting downshifting at slower highway speeds (45-55mph). With your current 35's, you'll be turning ~1254rpm at 55mph with 3.55 gears, or ~1387rpm at 55mph with 3.92 gears, which is very low. 4.56gears at 55mph would increase it to a more comfortable ~1614rpm.
If you have tall gears (3.55), you'll get worse mileage in the city running larger tires as it takes more to get the truck up to speed. It also allows for greater shifting at lower highway speeds. Going with a deeper gear, will allow better in town mileage and perhaps similar stock mileage on the highway as there will be less transmission shifting, and you should be able to hold an overdrive for a longer period.
