I can't give you any direct comparison for the Marlin but I just went through the same upgrade on a 2000 232 Gulfstream. I went from twin 150 OX66s to twin 150 four strokes. At cruise on the way out to the fishing grounds, I was averaging 1.2-1.4 mpg with the OX66s now I am between 1.8-2.0 mpg with the 4 strokes. I really see a big difference when trolling, the fuel economy has more than doubled. Last year with the OX66s, I would typically burn 115-120 gallons of gas for a day of fishing, now the most fuel I've used on any trip was 85 gallons. Good luck with your decision.