Horsepower, revs and engine size

[quo vadis]

I know running 2 Cummins 250hp electronic units burns 54 gallons per hour but that's it  

[Baitrunner]

The other factor with diesels is torque.

 

A larger engine will generate a lot more torque so could be much higher geared, thus although less power (hp) will run slower and turn the prop faster.

 

As mentioned earlier - I run a 3litre diesel Audi A5 and it will deliver 50mpg being thrashed, vs a 1.4 litre Focus I had which had to be thrashed to go and returned a lot less MPG.

 

I reckon there is an optimum size engine for the size of boat you have so it's important to match the 2