When I towed back from Portland Rose Cup in June 2003, I got 16 mpg with my dual axle open trailer, VW Wabbit, 8 tires on the rack, and the entire bed of my 1998 Dodge 2500 4x4, Quad Cab, Turbo Diesel, filled with tools, parts, etc. Tonnau cover also helps, as well as the 4" turbo to tail-pipe exhaust, and the SuperChips programming of the ECU.

Sweet spot for it is about 1,900 rpm, at about 64 mph

You hardly know the trailer / car are behind you.