My frist bumper pull had a very square front. Had to carry a canoe on top of my Suburban back from Castro Vally, CA for daughter and son inlaw and exptected my mileage to head south. To my surprise it went up. Did some research and found that the Sought Australia Caravan club had a member who did additional research so built a roof top deflector close to their recommendations. First trip, one I had made several times and kept track of mileage each time, and my savings was about 10%. What a surprise. Did some modifications and it went up to 12% to 15%. Happy camper!. Next two trailers were more aerodynamic. Minimal savings so abandoned it with my North Trail. Now I am looking at the 'bustle' you now see on some long distance semis. Could be another large improvement in mileage. The new 5th wheels are quite aero dynamic so not sure what savings may be made with a roof top deflector. Check out South Australia Caravan club and see if they have any recommendations?