I've been taking data for about a year and a half of what the vehicle estimates my mileage is and logging the miles driven and gas pumped. It is obvious from
This is the distribution (histogram) of the reported miles per gallon from the vehicle:
This is the distribution of the difference of the difference of the reported MPG and calculated MPG (from miles driven/gallons of gas):
Mean reported MPG is about 30.4 MPG; mean calculated difference is about 2.3 MPG.