twin F225A's Fuel Management

[sailorboy]

Finally got the twin F225A running enough to check the fuel management gauge. on the 2nd tank refill the system was reading 12% more fuel burn than actual. The calibration switches ONLY allow for +3% and -3%, my last boat had single engine and was spot on with out adjustment. Any suggestion on what may be going wrong?

[Sequoiha]

Is the speedo reading correctly, the 6y5 gauges use speed input as well,

[sailorboy]

I was only looking at the total gallon(s) reading. The speedo is a seperate gauge. Never switched to gpm. However the speedo is reasonably accurate

[rodbolt17]

is the FFM actually wired to the tach or speedo ?

[sailorboy]

FMM wired to speedo, can be connected to GPS if that is better source,, ohh.
New sending units, new cable,
Thanks

[sailorboy]

BTT..
Any ideas on how to correct the 12% error