2006 Gas Mileage: MFD vs. Purchase

In 2005, I created a spreadsheet to compare my fuel economy for the year between that computed from a log of my gas purchases versus a weighted average from the consumption display on the MFD (I recorded the miles and MPG every time I put gas in the car). This spreadsheet was the subject of the thread at this link. That thread discusses how the weighted average was computed.

I made my last gas purchase for 2006 today and have now completed the 2006 spreadsheet based on the final version of the 2005 spreadsheet (file attached).[attachmentid=6055]

I travelled a bit less this year (12958 miles vs. 14439--I didn't take a driving vacation this year) and used 37 fewer gallons of gas (273 vs. 310). I improved on last year's gas mileage (47.5 vs. 46.6 via the log and 48.3 vs. 47.9 via the MFD). Note that the log differential is more impressive and the difference between the log and the MFD decreased from 2.73% to 1.58%.

The 2006 difference between the two ways of computing mileage comes from the MFD showing 2.5 fewer miles and 4.3 fewer gallons of gas. Part of ther reason that the MFD numbers are lower is that the MFD does not reset immediately after a fill-up, but only after driving a short distance (usually between 0 and 0.25 miles). I think that the 2006 numbers are closer than the 2005 numbers partially because I changed the way I add gas. I should note that most fill-ups occurred at the same gas station, although at different pumps there. However, in past years, I tried to come close to topping off the tank, usually adding gas until the clicks occurred with very little gas being added (usually more that 1 gallon after the 1st click--no spills happened). However, this year, I was much more relaxed about it, usually forcing only 2-3 clicks after the the 1st click off. My hypothesis is that the pump accuracy degrades when small volumes of gas are added, leading to a slightly higher listed total (and charge) for the gas than was actually pumped.