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NMEA Reported MPG Accuracy?

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  • NMEA Reported MPG Accuracy?

    Hey there.

    I have the DF90A hooked up to a Lowrance Elite 7ti. Very nice!

    I see the gauge reporting MPG and have to wonder how this is calculated? Is this info only from the Suzuki assuming zero slip (although it doesn't know the prop specs), or does it use the Lowrance GPS speed dynamically to figure the MPG?

    I am happy with the reported 5-6 MPG while crusing along which would be about twice what I was getting with our Johnson 70 HP at just under 3MPG average.


  • #2
    The short answer is that 2 sources of data are brought together in the Lowrance so it can calculate mpg:

    The first bit of data required is how much fuel is being burnt at that instant. This is be reported by the ecu as gph. This data is provided to the Lowrance by the Suzuki interface and is drawn from the engine's ecu, which is constantly adjusting the amount of fuel that it is telling the injectors to inject into the cylinders.

    The second bit of data is speed. The Lowrance can be set to use either water speed (from either the pitot tube or a transducer paddle wheel) or SOG (speed over ground) from its GPS.

    So by combining mph and gph, the Lowrance easily calculates instantaneuous mpg. It changes constantly.

    Obviously the most accurate mph speed source to use is SOG from the GPS.

    Can i suggest that once you have set your system, you check the accuracy of fuel data by doing the following:

    Starting with a full tank and the system set to full, go fishing and burn at least 1/4 tank. The more the better. Come back and refuel, try to use the same gas station and pump as when you last filled. Fill to the same "fullness" as last time. (The reason to use the same gas station and bowser is that the angle that the boat is sitting can affect how much fuel goes into your tank).

    Then compare the actual amount it takes to fill with what your network tells you you have used. If they are the same - great! Your system is accurate and will stay that way. Use the refuel menu to reset the tank to full and complete that process and you are done.

    If the 2 figures vary by more than 2-3%, go into the Lowrance refuel menu, select "set to full" and it will then ask if you want to calibrate. Say yes, and then it lets you input the amount you added at the gas pump. Do that and complet the refuel processs in that menu. This calibrates the system so that next time, you will be even closer to 100% accuracy. You may have to do this calibration 2-3 times to get it close to 99-100% accuracy, then leave it alone.

    Last edited by Moonlighter; 06-07-2017, 02:00 AM.


    • #3
      Excellent! Thanks for the info, Moonlighter. Yes, I am working on burning fuel to see how accurate it will be. I need better weather to get this done sooner. I will print your info for the next fill up.

      Thanks again...