I'm brand new here & I just purchased a Laramie Long Horn Ecodiesel last weekend and I have roughly 350 miles on it so far in a mix of 50-50% city/highway driving. I'm a little disappointed as I'm averaging 20 MPG. I had hoped the highway would be better, but I realize it had less than 5 miles on it when I bought it. When will the mileage begin to climb for the highway?