I agree with milage and speed. as said above if you just tow at 60 youll get the best gas milage you can. I recently tested to see what I would get at different speeds then tailgate up tailgate down.
the best I got was 16.3 and that was at 65 mph tailgate up from amarillo to ft worth. my normal around town is 13.4 or so, if I keep my foot out of the pedal I can squeeze 14 or so and I have the little v8 (4.6)

Michael