We've got to assume you're talking about your header subject GPH vs MPG
- not miles per hour as you probably mis-typed above.

GPH is irrelevant, even with a trawler, hull speed type boat (of course
in mho). What does it tell you? How many hours of fuel you've burned,
how many hours you might have left? You'll still need to know how fast
you have gone or expect to go to determine where you can get to with
the fuel you have.

The ONLY reason I can guess that GPH is even used is that it has been
the only thing that could be measured with a normally available,
affordable gauge up til now. Fuel flow meters are common technology and
have been in use for ages. They can only tell GPH and that is why
people have been using that measurement. Everyone I've boated with
whether in a planing hull or displacement hull has a GPH flow meter,
but uses that info to mentally convert to MPG using their GPS or
speedometer reading. Yes, if the boat is a pretty steady traveler,
they could short cut by just looking at GPH, but when push comes to
shove (e.g., can we make it "there"), it is MPG which will tell you
what you need to know.

My only reason to have a flow meter on my boat (GPH) is to ultimately -
through a mental calculation- determine the most efficient speed, in
other words, the speed that maximizes MPG.

Nowadays, it becoming more common have the flowmeter GPH input to the
GPS's MPH measurement to give you an MPG reading on your GPS screen.
If I could afford a new unit, I would definitely get that feature -
because again, that's the only reason to measure GPH in the first place.