I have a new aircraft with a Rotax 912is Engine and Garmin G3X touch system. I have about 65 hours on it in the last two months. When I fill the fuel tanks between flights and compare the fuel consumption displayed on the Garmin fuel calculator to the actual gas filled, it is always inaccurate by a little over 10%. The aircraft is actually burning 10% more than indicating. In most Garmin G3X installations, there is one or two fuel flow transducers with a calibration K-Factor. The Garmin system has means to adjust the calibration of the fuel calculator display by adjusting the k-factor. 
In my research trying to fix the inaccuracies, I learned that the Rotax ECU sends data to the Garmin G3X through an interface. The Rotax engine does not have an actual fuel flow transducer. It sends "theoretical" data based on the fuel injectors pulses and pulse width. Based on the data from Rotax, Garmin simply displays what Rotax sends it. The calibration menu screen is missing and no one has been able to advise how to get this menu page on the system, or any method to correct the fuel flow calibration error. 
This is frustrating. I would not be as worried if it over estimated the fuel flow, but the fact that it under estimates is troubling. I fly long over water missions and I really want accurate fuel consumption data. 
A search of this forum history turns up a couple of other pilots who also reported the 10% underestimating of fuel flow. Those forums go back about four years ago and no solution had been discussed. 
I am reaching out to you for answers. Let me know if you are seeing this same issue and if you know of any solutions. 
Much thanks in advance. 

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