[bestclimb]

when you are running there will be a most efficient indicated speed. Some speed up or down from that depending on what the current is doing will yield the best MPG so some GPS information would be useful but a close enough approximation can be made just by knowing the current speed which can be learned by looking at it. Drifting down stream will be 999999999mpg but that is not useful information.

The OP already has a speed sensor. That will give a good deal of information if the signal is compatible, if not *shrug* the speedometer with knowing the GPH is nearly as good.

Increasing speed over that needed to be on plane is not likely to increase MPG. Yes there is less hull in the water but the force needed to lift the weight of the boat up on step is the same at 25 as at 50mph. The smaller contact area is doing the same lifting by hitting a smaller number of water molecules faster so the drag is about the same. Increasing speed will increase the amount of drag from the lower unit.