Ah, that will be dictated in a large part by the cruise speed, hull cleanliness, whether the engine is tuned and sea state. Mileage will definitely vary. Keep in mind the rule of 3rd's. 1/3 to get there, 1/3 to get back, and 1/3 in reserve. If yo urun for a while, you'll get feel for average consumption rates. sorry, I couldn;t be more help.