On a gas engine the rough formula for estimated fuel consumption is:
Est. Fuel Consumption (@ WOT) = (HP x 0.425)\6.2
0.425 = Estimated lbs of fuel burned per 1 HP
6.2 = Average lbs/gal of gasoline (Note that gasoline expands in higher temps and contracts at lower temps. Since a Gal. is a unit of volume and LB is consant, the weight a Gal. will vary slightly due to temp. variations. 6.2lb is an average weight of Gas)
HP = 310
Max RPM Range = 4200 - 4600
7.4 MAG MPI
HP = 385
Max RPM Range = 4600 - 5000
I strongly suspect that you have the 7.4 MPI and not the 7.4 MAG MPI.
(310 x 0.425)\6.2 = 21.25 GPH @ WOT. You can roughly estimate the decrease in fuel consumption in correlation to the decrease in average RPM's. So, if the Max RPM is roughly 4400, then if you are running at 3300 RPM, then you are at 75% of the max rpm range.
21.25 GPH x .75 = 15.93 GPH
So at 3300 RPM's the total fuel consumption for both engines would be approximately 31.875 GPH. Again, these are rough estimates. How well the engine is in tune, ambient air temps and sea states will all cause variation in the actual numbers. This should be close enouugh for jazz, blues and government work. Also keep in mind that the fuel consumption is a curve and does vary slightly between certain RPM ranges. So 50% RPM will not be exactly half the consumption of WOT. However (again) it will be close for general consumption and range estimations.
I hope this bit helps.