I just did a 1200 mile trip in a new 2005 Sport Trac. It has a 4.1 rear end. Cruise was set at 78 and over interstate with rolling hills, 2 adults and luggage. I got 17.8 MPG. That seemed a little low to me since our Mercury Gran Marquis (4.6-V8) normally gets 24 on the same trip. Is this...