Nerd Alert: The Obligatory Gas Mileage Chart

TB makes fun of me for religiously tracking the gas mileage of my 1997 Mazda 626. I have been doing it fairly consistently, using Microsoft Excel, since mid-2004. She thinks only nerds track such meaningless information so, in her honor, I post my car’s mileage calculations.

Keith's gas mileage
Mileage Definitions
  • The row highlighted yellow indicates a roadtrip with extended driving over a relatively short period of time which skews the price per day average of that entry.
  • The rows highlighted green were periods when we were on vacation which also skews the price per day averages, just in the opposite direction.
  • The double-lines indicate points when I changed from 89 to 87 octane fuel (and back). I explain this later.
  • The heavy, dotted-line signifies the move from our previous home which reduced my one-way commute from 17 to 5 miles.
  • I subtracted the days I was out of the country from Aug-Sep from the 7/26/2006 entry.
  • The 9/13/2006 entry is missing values because I am unable to supply the “Miles” and “Days” data until my next fill-up.

I have excluded the “yellow” and “green” anomalies from the analysis which follows.

Table comparing 89 vs. 87 octane Table comparing Far vs. Near driving distance.
Table 1 Table 2

Table 1 shows the effect the switches between 89 to 87 octane had on price per gallon (PPG), miles per gallon (MPG) and price per mile (PPM). The 38% increase in PPM ($0.08 average with 89 vs. $0.11 average with 87) immediately suggests a switch back to 89.

However, the PPG average of 87 octane is 18% greater than what I paid on average for 89 octane. In reality, 89 octane is always more expensive than 87 octane on any given date. Therefore, any conculsions drawn based purely on the above PPM data would be largely unreliable.

I also see, from Table 1, my car seems to prefer 89 octane over 87 octane as the MPG averages show I get about 2.3 more miles per gallon with 89 which is a 9% improvement over 87. Since the price of 89 is always within 9% the price of 87 (usually on 10-15 cents more), the MPG data indicates I would be better switching back to 89.

Of course my commute was much longer during the period I used 89 predominantly so the additional MPG efficiency is likely a factor of the amount of highway vs. city driving. The only way to be sure was to switch back to 89 octane and measure the MPG with my shorter commute. I did just that beginning on 6/30/2006 hoping the PPG would also stabilize enough to make such comparisons meaningful.

As you can see from the raw data, the 6/30/2006 and 7/26/2006 fill-ups with 89 octane had very similar PPG, miles driven, and days covered compared to the 5/3/2006 and 5/23/2006 entries with 87 octane. If 89 octane is better for my car I would expect to see improved MPG and PPM when comparing the two sets of entries. Also, after some quick math, I can see 89 cost 3.2% more than 87 but increased MPG only 1.5%. My engine doesn’t knock when I use 87 so there is no benefit for me to pay more for 89 to get¬†essentially the same results.

Table 2 shows my price per day (PPD) average dropped $0.09 (6%) once we moved closer to work. My car’s average MPG of 28.2 when doing mostly expressway (far) driving is pretty good. Now that we live closer to the office, I no longer do as much expressway driving so my MPG has dropped (10%) as one might expect. Living closer to the office means I get fewer miles per gallon but go much longer between refills.

Looking at the chart below, it is interesting to see how the price of gas has changed over time and at different points during the year. The chart confirms prices are lowest in the winter and highest in the mid-to-late summer. I wonder if there are any road trips worth planning between December and February.

Price of Gasoline from Jun 2004 until Sep 2006

If being able to make informed decisions that save me money make me a nerd, so be it. In case you’re wondering: To maintain consistency, I usually get my gas from any nearby Unocal 76 or Chevron after the low fuel light comes on and fill up without topping off.

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