I have a new (second hand) boat installed with a Garmin Echomap 9, which I love and get on really well with.  

The inboard Merc has a Garmin GFS10 fuel sensor installed and connected via NMEA.  As the original old analogue inboard fuel tank gauge is a bit temperamental, like so many of them, I'd like to really more on the digital version based on calibration/fuel flow.  (BTW, I do generally still rely on the old fashioned way of simply making sure I have plenty in the tank before I venture out)!

However, the previous owner has set the fuel tank capacity incorrectly.  No matter how I try, including following the manual, the unit refuses to allow me to chance the total tank capacity.  I can get to the screen that allows me to make the change, but it won't allow me to save it and simply goes back to the original number.

Anyone else had this?  Any clues?

Cheers!