When I had a Sunbird with a 135 HP Optimax outboard installed, it had a Mercury smart guide installed in place of the regular speedometer. It had a speedometer, fuel flow g/h, gallons used, functions, among others.
I found it helpful to find that sweet spot for speed and economy. Also, the fuel used gave a better idea of fuel remaining, not that I would want to run it down to one gallon left.
Dave