How fast 3 mph or 70 mph, somwhere in between? We're not just giving you a hard time, but there is literally no way to answer your question. I will make a stab at it though. SWAG Alert, SWAG Alert!!! If this were a 25 footer capable of around 6O mph with a gasoline I/O drive (Merc 425) run at about 35 MPH I think you will get 2.2 nm per US gallon of gasoline at that speed . . .