RE: outboard motor mpg
GPH is more a term used to estimate WOT consumption and to get an idea of a motors true output (with carbed and EFI motors, they typically consume 10% of their rating at WOT in one hour, example a 200hp motor would use around 20gals at WOT for one hour).
As for mpg per motor, the largest variable is the type of boat it is used on. Obviously a 150 Mer EFI is gonna get better fuel mileage on a bassboat that it pushes too 60mph than it would on a 24' pontoon that barely goes 20mph. Greater distances covered with the same amount of output yields greater efficiency or fuel mileage. Thus the fact that hull design (which is a combination of both hydrodynamics and aerodynamics) has as much or more impact on fuel mileage than does the motor itself.
My personal boat is among the most efficient boats known. It is an Allison XB2003 ProSport that is 20' 03", the barehull weighs around 1k#s and the motor is a 225 ProMax SS (which is a EFI motor built by Mercs Hiperf division and actually dynoed 242 at the prop which means it made about 260-270 at the head). Most V-6 20'+ bassboats will get around 2-3mpg at 3/4 or better throttle. I average between 5.5-6.5mpg so call it 6. That is twice the fuel economy of most similar boats, here it's STRICTLY a matter or hull efficiency. (A couple years ago a major magazine tested a Allison SS2000 with a 115 Fict Evinrude and got a maximum mpg rating of 12, and the boat would run over 80 on the top end. Basically unheard of in anyother types of boats!) Another benefit to hull efficiency is greater speed with less horsepower. My boat will run over 90mph. Most hulls that same size with the same motor will be doing good to run 75mph. That extra speed yields increased fuel effeciency.
The new DFI motors like Mercs OptiMax, Yammies HPDI and Bombs Fict are quite fuel stingier than their carbed and EFI counterparts. Getting 1 or 2 more mpg doesn't sound like much. BUT when you realize you can be talking about a 2mpg boat now getting 2.5-3.5mpg you are seeing differences of 25-75% better fuel economy. For the more performance oriented boater they hold more promise than the current technology of 4stroke outboards.
As for the 4strokes, they have some advantages like quieter, smoother and typically better fuel economy at low speeds. The dis-advantages are their extra weight (which creates more drag and decreases efficiency, example: 225 Optimax is 520#s, 225 Verado 630#s) they typically have no better and maybe even worse fuel economy at 3/4 or more throttle compared to similar output 2strokes. Another overlooked thing about the 4strokes right now, you can run a 2 stroke until it blows and rebuild most of them for a couple thousand at most. No one really knows what it would cost to rebuild the 4strokes, but most estimates put it at basically scrapping the engine and starting anew.
3.3mpg with a 115 is in the moderate/high range for fuel consumption for motors that size.
RA