My fuel management gauge is not reading properly. I am running twin yamaha 200 hpdi engines. The engines and the gauge are new in 2007. Since that time and up until three months ago the fuel management gauge was accurate to within a few gallons. Now, the gauge consistently reads below the actual usage (i have tested it on five separate occasions). The formula is exactly the same where it reads 2/3 the amount of that i actually consumed. I realize there could be several explanations for this (and possibly others i have overlooked):

1: the sending unit on one of the engines is not working properly; (but then it would read 1/2 of what it should, right)
2: there is air getting in the fuel line causing the sending unit not to work properly; (i checked all the connections)
3: the fuel management gauge itself is not working properly and needs to be adjusted;

The fuel management gauge needing adjustment is the easiest and simplest explanation but I do not know how to adjust it...help

Optional Information:
Make (of engine): yamaha
Model (of engine): hpdi
Year: 2007
Horsepower: 200

Already Tried:
I checked all hose connections and tightened; i put a clear hose piece in line (just behind fuel filter/separator) and checked for air bubbles (could not detect any) Have tested four times: 1: i consumed 15 gallons while the gauge read 10 gallons; 2: i consumed 165 gallons when the gauge read 110 gallons; 3: i consumed 200 gallons and the gauge read 135 gallons; I checked for any fuel leaks and I found none....