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Beware of dashboard "MPG" meters

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  • Beware of dashboard "MPG" meters

    I have a liquid fuel , 2014 car, with the "mpg" in real time, and accumulated, display. It think it uses the assumed flow thru the fuel injectors by time and pulses, as computed by the ecu.

    I kind of got suspect that it was not accurate, so I did an actual gallons and miles check over several tanks. It's about 20% off, Real mileage is about 22, meter says avg 24.8. The only variable could be off , if the fuel pressure is higher than the computer sees. Dont know if it assumes a given fuel pressure, or has a pressure input from a pressure sensor.

  • #2
    Re: Beware of dashboard "MPG" meters

    I agree

    It uses flow rate which is calculated using Pressure, and Injecting Timing. On gasoline it can be very accurate with newer vehicles however, it seems to me that all those calculations go out the window when running on CNG since the vehicle doesn't know the calculation for fuel density, pressure, or accurate timing.

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    • #3
      Re: Beware of dashboard "MPG" meters

      I was surprised to find that GM disabled the fuel economy section on the dash display for both CNG and gasoline on my 2013 pickup. I suppose that the calculations would be off but it would be nice to have at least the gasoline information shown so that I can complain how bad the fuel economy on the 6.0 liter engine is. However, I was surprised to find that hooking a little scan tool to the OBD II diagnostic port yielded fairly close calculations on either fuel. I was also surprised to find when connecting a Tech2 to the truck, there is very little information coming from the CNG system compared to my older vans and Cavaliers. Tank pressure (which would be nice to know) isn't there---just a percentage of fill. There are no run times or miles driven on alternative fuel like was found in the older vehicles.

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      • #4
        Re: Beware of dashboard "MPG" meters

        Originally posted by siai47 View Post
        I was surprised to find that GM disabled the fuel economy section on the dash display for both CNG and gasoline on my 2013 pickup. I suppose that the calculations would be off but it would be nice to have at least the gasoline information shown so that I can complain how bad the fuel economy on the 6.0 liter engine is. However, I was surprised to find that hooking a little scan tool to the OBD II diagnostic port yielded fairly close calculations on either fuel. I was also surprised to find when connecting a Tech2 to the truck, there is very little information coming from the CNG system compared to my older vans and Cavaliers. Tank pressure (which would be nice to know) isn't there---just a percentage of fill. There are no run times or miles driven on alternative fuel like was found in the older vehicles.
        I didn't know GM did away with it. Seems like a simple task to integrate live data from the CNG system into the CAN port. Fuel calculations should be easy as well. The ecu knows how much fuel is running through the injectors and the speed. Should be a simple formula and display on the dash. After all they are just estimates. Would be nice to know a ball park.

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