Incidentally, here is an example of my rationale which I strongly encourage anyone to correct my mistakes or oversights.
Distance: 30 Nautical Miles (NM)
Fuel Consumption/hr @ 2400: 2 Gallons
Speed @ 2400: 8 kts
Speed into Head Sea: 6 kts
Speed with Following Sea: 10 kts
(30 / 10) * 2 = 6 gallons (0.2 kmpg)
(30 / 6) * 2 = 10 gallons (0.33 kmpg)
This a 66% increase in fuel consumption. Obviously a planing boat would see a less dramatic impact, however, the difference is there. In this case, calculating kmpg will potentially lead to a significant miscalculation.