I would talk to who you purchased it from. They might get you in the ball park. Their are alot of factors that determine GPH. Weight of your boat, type of motor, driver. You can get a cheap GPH meter for about $300.00 But you probably only really use it once to find your cruising speed. The consumption of my motor 200 merc 2 stroke carb. is about 15 gph at full throttle. Thats about 52 mph. I have 85 gallons of fuel onboard. So if I am doing the math correct. I should be able to go full throtle for 5 hours and that will get me 250 miles from where I started. Now where the heck is it ever going to be that smooth. Their isn't any place i go that far away. Becides I usually run about 15 -30 minutes on the big motor then I run the 15hp honda for the next 8 hours. I cant even begin to do the math how long that honda would run on 85 gallons of fuel. I would follow erik instructions and balance out the boat. I would make it as large as possible without making it transom heavy or bow heavy.