I just put my first Merc 300xs Optimax on a new 21ft Liberator cat. It only had 1 hour on it when I gave it to the customer and it still ran 99.5 mph. He told me the other day that he ran 190 miles on 33 gallons of gas. I'm rigging a new 30ft Liberator with twin 300xs motors now. I think it will get 3 mpg. Not bad for a 100 mph boat.