Normal highway tires if you are not planning on going offroad, and get them in stock size. That seems to have the greatest effect on mileage. General maintenance is the only other thing. Check your air filter as well, it can rob mileage if clogged up. You have the big tow gearing since you are at 2K RPMs at 70 MPH, so that is going to decrease the mileage some compared to the non-big tow titans, but it will really help when you tow a heavy load. Also, I didn't see it in your post, but are you 4x4? That will create a small dip in your mileage as well.