You MAY only have a 75 gallon tank! That happened to a friend of mine with a Sceptre on one Bahama trip! He gets to the gas dock with gage on empty, and, thinking he has a 100 gallon tank, dumps 2 gallons of oil in the tank before refueling. (This was 25-30 years ago when we all ran 50:1 premix). Tank overflowed at 75 gallons! A big blue cloud followed him around for the rest of the trip!
Floscan is the best flowmeter. A little pricey but aerospace quality and top notch customer support. We used their stuff in testing jet engines when I did R&D work at Pratt & Whitney. I installed one on my boat about 25 years ago and I could predict how much gas it would take within about 0.2 gallons!
An even better option, if your engine is fuel injected, is to just use the factory system that ties into engine computer. The BRP I-Command system I have, similar to Merc's Smartcraft system, gets fuel flow from the computer and uses speed from a GPS antenna to calculate all sorts of stuff . . . instantaneous MPG (VERY handy for setting optimum cruise speed, trimming engine and tabs, etc.), fuel burned, fuel remaining, range on existing fuel. I was a little skeptical of the accuracy of the CALCULATED fuel flow, but took a long trip down the St. John's River last spring and when I refueled, the Floscan measured fuel burn matched the calculated fuel burn and actual fuel used within 0.1 gallons! The photocell finally died in the Floscan and it's so old that they don't stock that part anymore, but factory said I might be able to find one on the internet. No big deal as I'm now in a believer in the I-Command system!
It's also possible to calibrate the gas gage, which I did when I first got the boat. Make sure boat is level or at least at the same attitude as when in the water, and hook up battery charger so voltage level is same as when engine is running, as that will affect gage reading. Gas gage is essentially a voltmeter that reads voltage drop across the resistor in the sending unit. I put some white vinyl tape across face of gage, drained tank, refilled from 6 gallon cans, and put a tick mark on the tape adjacent to the needle after every 6 gallons. It worked pretty well and I could predict how much fuel it would take within 2-3 gallons.
If you don't calibrate the gage, I agree with Gillie . . . about the only thing you know for sure with a gas gage, if needle is moving, that there is SOME gas in the tank! If needle doesn't move, you don't know anything!
Denny