FWIW, our small express cruiser has three "gears".
First, coasting ... slow and efficient. Second, cruising ... fast and equally efficient. Third, WOT or 25GPH or $100/hour ... but fun, in the right place at the right time. Here are the specs:
1st ... 7mph/6kts at 2.5mpg
2nd ... 32mph/28kts at 2.5mpg
3rd ... 47.5mph/41kts at 1.8 mpg
Yes, it sounds crazy to all of us with other style boats, but I remember those type numbers from our Cobalt when we were on the lake.
I did find a boat we own that gets good fuel mileage....
Engine: Yanmar 150 hp.
With one person
4000 rpm, 38 knots, 10 gal per hour, 3.8 nmpg
3000 rpm, 25 knots, 5 gal per hour, 5 nmpg
2000 rpm, 15 knots, 2 gal per hour, 7.5 nmpg
With four people and load
4000 rpm, 34 knots, 10 gal per hour, 3.4 nmpg
3000 rpm, 20 knots, 5 gal per hour, 4 nmpg
2000 rpm, 12 knots, 2 gal per hour, 6 nmpg
So what kind of boat could this possibly be?