As jleonard noted, one or maybe both of your numbers is incorrect.
But in any case, here are a couple of data points from our 2006 Mainship 34T with the Yanmar 370 engine. The fuel burn and speed numbers will be close for the Cummins engine, but the rpm numbers will be lower for the Cummins by about 10%.
1,700 rpm, 3 gph, 7 kts
2,800 rpm, 12 gph, 12 kts
FWIW those are the best two points to operate that boat at. The 7 kt speed is near displacement speed and gives you good fuel economy. The 12 kt speed is a bit over the hump and any faster is loading the engine too much IMO.
And also FWIW the Cummins QSB 330 hp engine is rated at 330 hp at 2,800 rpm. I wouldn't cruise it any faster than 2,500, but I won't tell; it is between you and the owner.