Fuel burn. Optimax 175 vs Merc 115 4stroke

[Basketcase]

Hey guys. I have an Opti 175 on my Sceptre 20 and it is pretty awesome. I was planning to put the Merc 115 on my 20 CC when its done. Any idea what the fuel burn difference will be between these motors? I'm wondering if I made a bad decision putting a 25 gallon fuel tank in the CC. When I made the decision I was driving a 17.5 aquasport with etec 90 and was using about 15 gallons of fuel for a weeks worth of boating at the island house. I replaced the aquasport with the sceptre and needless to say I burn a LOT more fuel. I decided on the small tank in the CC because I figured 50 gallons of fuel would go bad before I could burn it.

What are your thoughts? Will the 115 on that boat burn more like the etec 90 or the opti 175? Somewhere in between?

On another note. I fricken love the layout and function of the sceptre way more than the Aquasport CC I had. (obviously the ride is not even comparable) Not sure if it was just size, or the huge dance floor in the back on the sceptre. It really is hard to get motivated to finish the CC when I'm loving the Sceptre so much.