Fuel mileage

[Seacrets]

I've got a Navman 2100 and I'm still trying to setup the dang thing. *I relocated the probe to a more vertical installation and it said I burnt 27 gallons and I fill up and I can't get more than 15.5 gallons in the tank. *It's programmed for U.S. gallons. *I'm about ready to install a new tank sending unit with a gauge mounted on the tank fitting and replace the inspection lid with a clear plastic one and get my fuel reading at the tank. *Kamikazee, your's might be set in liters and not U.S. gallons. *There's three settings, Imperial gallons, liters and U.S. gallons. *Just a thought.