Let's do this - take a Grand Banks 42. They were available early with a single 120HP Ford Lehman; twin 120's; and later twin >300hp Cat 3208s (1980's); and finally, a more recent GB42 with twin 375hp Cummins B-Series (Tier 3/Common Rail - T/A) engines.
At 7.5 kts, I will
guess the following as 'directionally correct' for continuous running in open water (+/- 0.3 gph):
- Single 120HP - 2.25 gph
- Twin 120HP - 2.75 gph
- Twin 3208's - 3.0 gph
- Twin Common Rail - 2.50 gph
As comparison, I was aboard a friend's Willard 40 with a John Deere 4045TA (Tier 2 at the time - 2001 build) from Long Beach to La Paz MX, about 1000nms and burned under 1.7gph in open ocean conditions.
I'm sure there will plenty of feedback on the above guesses, but I think you're looking for relative comparison.
Peter