Difference between displayed and actual MPG

Ever since I bought my Prius, nearly 7 years ago now, I've tracked the fuel consumption on Ecomodder, which calculates actual MPG from gallons replaced and distance driven. I have also recorded the displayed MPG for each tank. At just over 90,000 miles, that means 178 tanks with calculated and displayed MPG data.

I put it all in an Excel spreadsheet and calculated mean, median, quartiles, and outliers for actual difference and percent difference for each tank. The outliers were few; 2 low values when the display showed less MPG than actual, an artifact of the reality that gas pumps are not 100% consistent. That left 176 "good" values for each statistic.

The results:



Mean = 3.04 MPG difference
Min = 0.9 MPG difference
Max = 5.3 MPG difference
Median = 3.0 MPG difference



Mean = 5.563% difference
Min = 1.44% difference
Max = 10.23% difference
Median = 5.58% difference

So, on my car at least the display averages +3.0 MPG high, or 5.6%.

 

Yep, exactly.

If anyone else has kept track of this, I'm curious what sort of consistency we would see e.g. if ~5.5% difference is typical. I see people post frequently with a number they think represents the difference in their experience, but I'm not sure anyone has actually calculated it.