The calculation for fuel (Gasoline) consumption
at WOT should be:
HP x 0.425/6.2 = Gallon per hour burn rate (PER ENGINE).
The challenge here is that gasoline expands and contracts with temprature, unlike water
which is relatively consistant. so it's really teh specific gravity of the fuel based on the temperature which dictates it's weight. At 68 degrees F, gassoline weighs 6.15lbs per gallon.
The Mercruier 454 had varied HP reports between 300 - 325 HP. I think 300 HP was a more realistic number.
300 (HP) x 0.425 = 127.5lbs of fuel
127.5 (lbs of fuel) \ 6.2 (lbs per gallon) = 20 Gallons per hour (At wide open throttle.
So you would have a 40 gal./hr. burn rate at WOT for both engines combined. Now, calculate the percentage of WOT. Let's say the engine
runs at 4000RPM at WOT, then theoretically, at 3,000 RPM
(75%) you would decrease fuel consumption by that amoutn relatively (30 Gal./hr.)
Now these are just rough numbers, and they WILL vary. Saying "well, I get 26gal. when the numbes say 30 is a minor varation. It is also based on a properly tuned engine. Many other factors will impact the final numbers, but this is a good start at the least.
How to Calculate Fuel Consumption in Boats | eHow.com
Calculating Fuel Consumption | Boating Magazine