2005 Gas Mileage: MFD vs. Purchase

I have just done an spreadsheet of my fuel log for 2005. I own a 2004 Prius purchased in February 2004. My stats for 2004 for not complete because my MFD was non-functional for several weeks. I should note that the numbers and explanations below describe the general method. In the actual spreadsheet, there is one odd row caused by accidently resetting the MFD between gas station visits. In addition, there is a high variation in mileages computed from gas purchases because of the my inability to fill the tank to a consistent level (there are a few cases of very early cutoffs).

Here are the 2005 stats as of my last gas purchase on 12/25:

Total cummulative mileage (from February 2004): 27611
Odometer Mileage in 2005: 14439
Total Gas bought: 309.701 gallons
Odometer/purchased mpg: 46.6

At each fill-up, I also recorded the stats from the MFD screen. I then computed an 'MFD gas used' stat by dividing the miles by the mpg shown. By dividing the total miles for the year by the MFD gas used, as computed, I can get the appropriate MFD mpg. Note that the miles recorded and gas used by this method should be slightly lower because the screen does not reset immediately on leaving a gas station.

MFD mileage: 14419
MFD gas usage: 300.931
MFD mpg: 47.9

Thus the MFD seems to have overstated the fuel economy by 2.73%.

 

I realized that a part of the explanation for the difference in the two MPG figures might be that mileages reported on the MFD were truncated to the nearest integer, while the odometer mileage was only rounded to the nearest integer. Thus, the MFD mileages were always less than or equal to the actual mileage (by 1 mile or less) while the odometer total was accurate. Since the average error was 0.5 miles, I created a block of adjusted data by adding 0.5 miles to each MFD mileage, recomputing the gas used, and all other figures based on these.

The net result was that total mileage now differed by only 3 miles (rather than 20 miles), and the calculated gas used now was 8.4 gallons less than that bought (old number was 8.7 gallons). In terms of MPG, the results were just about the same as before. The revised spreadsheet is attached.