With my 135 HP Optimax I had on my old boat, I also had the "smart gauges" that were available, one setting included GPH and total gallons used. (I almost wrote total gals used!!! that would have started some off topic quips
). Now, I have a carbureted Merc I/O and per Mercury, the smart gauges are not available.
I know I can get a flow meter installed, approximately $200 plus installation, but will wait until my boat disposable income can best be used on that luxury. It was handy to see at a glance where my best economy was, MPH VS GPH.