gas mileage will drop whenever you are in 4 wheel drive. I go by how the road feels not looks. But going off of what you described, I would keep it in 2wheel drive. You can shift into 4high at 60 mph so maybe just try to manage it as you go. But its about how you feel, be safe. You have a 4x4 for a reason and if gas is a second thought after safety, then keep it in 4wheel hi. but its up to you...