While the MPG of an individual tank of fuel may be distorted by variations in the exact fill-up level, your lifetime MPG is much less distorted (at some point, below .01 MPG).

FWIW, my manual calcs tend to be about 5% lower than the on board computer readings (approx. 16 MPG combined pretty consistently).

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2008 X5 4.8i, Jet Black, Tobacco Nevada leather & just about everything else (no vent. seats, no 3rd row)