MPG Question, cars display vs reality

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  1. MPG Question, cars display vs reality 
    #1
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    Soulassassin's Car Details
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    Hi everyone,

    On my car I reset the mpg and the trip meter every time I fill up and I always fill up at the same petrol station.
    On the OBC the MPG reads that I get 40 mpg, however when I work out the actual mpg (litres divided by 4.546 to get the gallons then miles done divided by the gallons) it works out that I get 36 mpg.
    That is 10% out!!
    So my question is, which of these is correct?

    a) The car is not measuring the correct amount of diesel into the engine

    b) The car is incorrectly measuring the miles done

    c) The car is deliberately programmed to lie about the mpg to make it look better than it actually is

    d) I'm being robbed by the garage of 10% of fuel
     
     

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    #2
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    I've never had a car give me the MPG correctly. If your math is right 36 MPG is likely your true MPG.
     
     

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    Soulassassin (27-11-2016)

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    The cars computer is almost invariably optimistic in my experience, regardless of the make of car.
    On some BMWs like the F10 you can adjust the on board computer to get closer to the true value calculated from brim to brim measurements via a hidden menu. Did this on mine and as a result the car is now rarely more than 1% out.
     
     

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    Soulassassin (27-11-2016)

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    Quote Originally Posted by Johnny Grabble View Post
    The cars computer is almost invariably optimistic in my experience, regardless of the make of car.
    On some BMWs like the F10 you can adjust the on board computer to get closer to the true value calculated from brim to brim measurements via a hidden menu. Did this on mine and as a result the car is now rarely more than 1% out.
    Mines a F34 335D, can you describe how you did this or link to where you got the info plz?
     
     

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    It's similar to this: BMW F01 F02 hidden menu diagnostic mode - mr-fix.info
    Each model is slightly different in the detail. For example on an F10 with the multifunction display, there is no need to enter the VIN combination.
    You're looking for the correction factor. Default setting is 1000. If your on board computer is 10% optimistic as you say, then you need to adjust the number upwards by 10%, so to 1100.
     
     

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    Soulassassin (29-11-2016)

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    Will give this a try when I get the chance, cheers.
     
     

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