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BIF said:
The MFD figure is only an ESTIMATION. Whereas the mathematically-calculated number is HOW FAR you drove on HOW MUCH gas.
The MFD figure is NOT an estimation. It is a very thorough calculation of fuel used in millisecond intervals. There is no estimating done. The calculation uses the actual injector on time for each fuel injection period, injector flow rate, and distance traveled to arrive at the calculation. The accuracy is within 0.1 mile per gallon for any amount of fuel used greater than 1 gallon. Less than 1 gallon the accuracy is limited by display resolution, offering up to 10% error. The MFD reading you see is within 2%, 1% typical, of the actual fuel economy achieved. It will take at least 20 fill-ups using manual calculations to begin to average out the affects of fuel fill variability. Even then, you will have up to 1 gallon of fill variation tank to tank as a result of the bladder and pump shut off sensitivity.
 
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