2009 Gas Mileage: MFD vs. Purchased

For the 5th year, I am again presenting a spreadsheet comparing mileage computations recorded from fill-ups at the gas pump with mileages displayed on the MFD. This year, I have combined the annual worksheets into an Excel workbook and added a summary sheet with the annual statistics and a grand total. Comments attached to the title of each spreadsheet are slightly modified versions of the Priuschat announcement for each year. Here is a link to the thread for my 2008 results.

Mileage for 2009, based of purchase logs dropped from 47.5 mpg to 45.8 mpg. As computed from the MFD (the Multi-Function Display screen), it dropped from 48.7 mpg to 47.1 mpg. I believe that this was primarily due to 2 factors:

1) The car tires were replaced in May,
2) My driving pattern changed radically on August 1 when I retired and no longer had a 40-mile round-trip commute to work. This resulted in an absolute drop in miles travelled (from 13563 to 10370) and gasoline bought (from 285.6 gallons to to 226.3). However, after July, most of my travel consisted of short round trips, with a large increase in under 10 mile trips.

The difference in mileage computed from purchase logs and from the information provided by the MFD was about 2.66%. This was very close to the discrepancy of 2.57% reported for 2008.

Note: in the thread that arose from my 2008 posting, Celtic Blue proposed using the odometer mileage be used to calculate the estimate of gas used using the mpg displayed by the MFD. I did a sample adjustment by that method for 2008 with the result being that the computed gas usage was 0.011 gallons higher than by the method I used. Although this is probably a better adjustment method, the small difference from the method used previously does not justify changing to it.