On our boat we have a fuel
gauge, but the tank is rather flat and as such the reading is not reliable.
I used the engine
manufacturers fuel consumption
chart and then the first few times we were out cruising compared the calculations with the actual usage (must run a consistant rpm
After a few trials I was able to comfortably calculate our fuel consumption
with remarkable accuracy.
I did these test durring several trips at different rpms.
As a side note I try never to run the tank all the way down (calculate a good reserve).