I'm sure this sounds like a silly question, but..... can you turn off the Auto selection off the mode selection knob on the Trailhawk?? The manual mentions that the Auto mode also reduces fuel consumption....as though you had the option to turn all modes off for better mileage. From manual:
AUTO: This four-wheel drive operation is a continuous operation, is fully automatic and can be used on and off road. This mode balances traction to ensure maneuverability and acceleration improvement compared to a car with two wheel drive. This mode also reduces fuel consumption, since it allows the disconnect of the drive shaft where conditions permit.