I have the 325 hp engine and used about 1/3 of the tank in an hour going between 3000 & 4000 rpm. I hope to get at least a days worth of use out of a tank of gas in my boat, that includes wide open, idle, stop, and somewhere in between. If fuel economy is going to be a big issue for you, the smaller V-8 may be a better option. But I don't think there will be a big difference in fuel economy between the motors.