The Garmin fuel gauge has been Fluid motions fall back fuel gauge for accuracy. And the fuel tank sender has been calibrated using full tank calibration. This is inaccurate with an irregular shaped tank. For years the analog fuel gauge and sending unit have been unreliable and inaccurate. Today we have electronic EVC monitors that convert fuel tank percentages to gallons. All engine manufactures of modern day outboards and inboards have some type of electronic monitor. Todays fuel gauges and sending units can be accurate. The prerequisite for an accurate reading is the boat must be in a level running position ( especially TUGS and CUTS because the sending unit is far aft on the tank ) and the sending unit must be properly calibrated. Again using full fuel tank calibration is not accurate!! It will give you a estimate at best.
Example: Jim Favors post. His Yamaha fuel gauge showed 1/2 tank. He took on 47 gallons and most likely had a few more gallons to go before the tank went to over flow (full). At a 10 gallon error that is just over 10% off. In the wrong way! If my fuel gauge is reading 10% more than I have in the tank that worries me. If the gauge is reading 10% less than I have in the tank. I know when it reads 1/4 of a tank I have at least 20 gallons + 8 to ten gallons. When boating, redundancy is always better. If you have two ways of checking fuel that are both known to be accurate with in a few % you can check one against the other.
Using the Garmin as a fuel gauge is an ok method but it is not always accurate. It relies on the captain to enter the exact amount of fuel pumped into the tank. If this entry is incorrect then the Garmin only has information based of fuel burn. It has no idea how much fuel is in the tank. It has no reference. What you entered is the only reference it has. If you know exactly to the gallon how much fuel is in the tank and the Garmin software does not have a glitch, or the N2K communication is not interrupted. You will have an accurate fuel reading. If you have an fuel tank sending unit that is properly calibrated then you have a secondary means of knowing how much fuel is in the tank. They should match up. If they don't match up which one do you rely on? For me I rely on the one that reads how much fuel is in the tank. Then I rely on my own calculations. My log book is my way of proving accuracy. Hours run, average fuel burn at a given speed = gallons used. I use the chart plotter fuel burn information because it is easy and seems to be accurate to a degree.
When I had my C26 the EVC Volvo backboned to the Garmin I found after two years of boating that the Garmin reading was off by 13.5 gallons. The Garmin showed I had 31.5 gallons in the tank. The Volvo EVC fuel gauge showed I had 57 gallons of fuel left. Which was right? I drained the tank to confirm which was right. After draining the tank into a marked 5 gallon container then pouring the fuel into a 55gallon drum I measured 45 gallons of fuel in the tank. Both gauges proved to be inaccurate. At that point I decided to properly calibrate the fuel tank sending unit. I have explained this several times on TugNuts. It is easy to do and should be done as part of a Quality Control item from the quality boat builder when the boat is delivered to the owner. Again, the full fuel tank calibration is worthless with an irregular shape tank.
After my calibration was done I relied on my fuel tank sender over my Garmin. I found it to be spot on. When I filled the tank I would use the Volvo gauge has my fill point. I would compare it to the Garmin with hopes that they both agreed. They agreed at the beginning but as time went on the Garmin would show a larger amount used. Its accuracy was repeating what I had experienced before. I would make adjustments to the Garmin based on what the fuel tank sending unit was telling the EVC. This helped maintain the Garmin's accuracy over time.
The link below illustrates the comparison Garmin reading fuel onboard and the EVC fuel sender with proper calibration. You will notice that the boat is operating at a slow speed and the boat is level in the water. The Volvo was calibrated by percentage of fuel and it will show the tank level as a analog reading or gallons. In the picture below it is showing gallons in the tank. In this photo both Garmin and fuel gauge agree within .7 gallons. I can live with that.
I'm not indicating that the Garmin is not a good tool to have. It is a great tool as long as all the holes in the cheese are lining up. I'm also not saying that the fuel sending gauge is 100% accurate but if calibrated properly it will give you a low percentage of inaccuracy. Something to use as a checks and balance and a redundancy accurate reading. When one is inaccurate then use you log book to compare. In my experience the Garmin over time becomes less accurate. The fuel gauge stays the same.
I'm back to old school now. My Pilot 34 mainship is direct injection. Fuel injected by the injector pump. There is no fuel burn monitor. I must use log book calibration and fuel tank level readings to know how much fuel is onboard and used. It has an analog fuel gauge which is old school and not accurate. Calibration is done by bending the float stick! I do have the most accurate fuel gauge onboard though. It is a pre measured stick with 5 gallon increments. Using this I know the exact amount of fuel onboard to the gallon. matched with my log book I know GPH and MPG to the exact amount. Old school but accurate, nothing to break no guess work. A 4' fiberglass stick!!!
I know this has nothing to do with the up coming Garmin update. It is my experience with electronic fuel readings compared to fuel in the tank gauge. Having both accurate is a plus when boating, Checks and Balances!!
