We are turning 1800 (verified with an external tachometer) rpm's. According to Yanmar at this rom we are burning 1/2 gallon per hour. We are moving through the water at 4.9 knots... A little less than 10 mpg.
Now we have pushed it up to 6 knots and we are turning 2300 rpm's. Yanmar says we should burn about 1 gallon per hour at this rate of turns for about 6 mpg...
These numbers haven't changed in 10 years of sailing Esmeralde and the fuel used pretty matches up to the burn rates Yanmar publishes on long trips (more than 24 hour deliveries etc).
We are pulling a Trinka hard dinghy and at the moment the alternator is putting out about 65 amps so those need to be factored in too.