I track all of my fill ups and compare the mpg shown on the display with the mpg calculated based on the amount of gas put into the tank. I reset the MFD MPG indicator every time I fill up. I know that there will be descrepancies between the two mpg calculations on individual tanks, but they should balance out over time. I have data for the first 11,500 miles on my Prius. I weighted the mpg values based on the number of miles and calculated an error percentage between the two mpg readings. My calcs show that the MFD mpg averages 1.3% higher than the true calculated mpg. Anyone else do that calculation?
As you can see from my signature mine MFD also runs a bit high. Since the "actual" bounces around quite a bit due to the bladder I think my true mileage is about 1 mpg less than is shown on the MFD. PA P
At just over 12,000 miles, my weighted average MFD reading (50.7 mpg) is 2.4% higher than calculated (miles driven/total gallons in = 49.6 mpg).
Based on the fact that ODO trip A/b readings stop at 1/10 miles and seem to round up, the % seems to make sense over time. I have a -.5 average MPG difference (My MFD reading is .5 Mpg higher that ODO trip B/Actual Gal) over my first 2000 miles
Without a truly independent measure there's no way to know how much of the discrepancy is due to MFD inaccuracy, and how much is due to gas pump inaccuracy. Almost everyone reports the MFD value as higher than the manually calculated value. By some odd coincidence any amount of this discrepancy which is caused by the gas pump would be in the gas seller's favor. 2% is within the error allowed by most states' weights and measures inspectors.
