Fuel Consumption

Does anyone know where I can get fuel consumption data comparing Merc 7.4, Merc 5.7 and Merc 5.0?

I've been told that 7.4's pushing a 31' Formula at cruising speed only consume about 5% more than the Merc 5.7's pushing a Maxum 3100 and it just doesn't sound right

Boat test reports are the normal place for finding that info. You won't find any 'published' fuel consumption data for most gase engines as the hull and gear cause the 'too many variables' situation. Finding the owner of a boat of interest and asking is another method.

The biggest engines will usually burn less fuel than the smaller ones in the same hull at the same speed - why? Because torque is required to spin the prop and is a fixed quantity for a given hull at a given speed. The larger engine will produce that torque at a lower rpm than the smaller one. When you compare different hulls with different engines, it is real hard to find a factor to normalize by.