I have a 41 foot boat (about 38'lwl) powered by two 220 hp Cats. The broker listing said "16 knot max". That may be true, but 13 kts produces more noise than I can bear, so I've never pushed it to "wide-open"
As to fuel consumption, I can get a reliable 1.7mpg at 8 knots--1600rpm. On the rare occasion when I run at 12kts, 2200, consumption falls below 1mpg. Fortunately, my background is entirely in sailboats, so I still find 8 knots to be fast.