July 22nd, 2009 - Back To That Weighty Question

Harking back to the post from June 25th, 2009 - "Lighten Up," I have been doing my own albeit unscientific research about my proposed question: "How much difference does filling your gas tank up to only half full make to your miles per gallon?" i.e. Does the weight of a partial tank of gas make an appreciable difference in your vehicle's fuel efficiency?
After simple testing for nearly a month, here is what I have found.

I dug up my MPG records from the last two years (I know, you're choking on my geekdom) and after throwing out 2007 because it had dealt with multiple cars, I figured it would be "more accurate" to compare MPG at the same time of year, as MPG dropped steadily with the onset of colder weather.
The numbers I looked at were the 2008 June & July, 2009 June before "the experiment," and 2009 June & July during the experiment to present. (It's still going on)
I'm not sure when I had an oil change during this period in 2008 (another fairly large influence on gas mileage to the positive) so I guess I'm not all THAT geeky. Actually I could look it up, but I figure since I haven't had one in the two months we'll consider that a non-factor.

Okay. Let's turn to the "hard numbers:"
Are you on the edge of your seat? I am on mine.

During the 2008 June - July period, this was my MPG per tankful for the 2000 Focus wagon, not paying attention to when or where I filled my tank, duration of the trips, or what I was carrying, etc:
28.16 MPG
29.94 MPG
30.87 MPG
25.65 MPG
30.40 MPG
29.56 MPG
29.96 MPG
29.22 MPG Average

During the 2009 period this June before the experiment, this was my MPG per tankful for the 2000 Focus wagon, not paying attention to when or where I filled my tank, duration of the trips or what I was carrying, etc:
30.79 MPG
29.91 MPG
31.83 MPG
30.83 MPG
30.40 MPG
29.56 MPG
29.96 MPG
30.84 MPG Average.

During the 2009 period this June & July during the experiment, this was my MPG for the 2000 Focus wagon, paying attention to only filling my tank up after the "low fuel" light came on, and filling only to precisely 7 gallons (approximately 1/2 tank) of fuel:
30.77 MPG
34.51 MPG
35.07 MPG
33.28 MPG
33.43 MPG
33.41 MPG average.

My results show (and your results may vary, especially through the use of Irish whiskey and Greek symbols) that my vehicle's fuel efficiency DID in fact go up, and far more than I expected.
Interestingly, the two highest MPG values occurred during a 440 mile round trip, where the car was heavily laden with luggage. I did however keep to my schedule of only 7 gallon fill-up stops.
All the other mileage is "normal," about town and in-town commute mileage, and I was usually re-filling at the same gas station.
Admittedly, this is not a lot of data. However, HOW-EVER, a +9.23% increase in MPG over a month to me is not a mere bagatelle.
Coupled with the fact that I have NEVER EVER in the past gotten over 35 MPG under any circumstances, it gives one pause...
Anybody wanna buy some back seats for a 2000 Ford Focus Wagon...? Spare tire? Jack? Maybe if I take one lugnut off each wheel... hmmm.


buthidae said...

That's a neat result. You have to wonder how much of that mileage gain is actually the result of reduced vehicle weight, and how much might be due to subtle improvements in your driving habits because on some subconscous level your testee self was aware of the results your tester self wanted.

dignature said...

Good point. They say that people that drive Prius' for example with the "MPG meter" do better just because they are more conscious due to their constant monitoring of it.
I guess if I continue gathering my results, the long term perspective will be more apparent and some of that "novelty" will have worn off and my driving habits and everythig else will even out.
The interesting thing to me was the long over-the-road drive with a lot of weight in the car. I thought that in itself would cancel any gain through carrying less fuel. I will be interesting to try on the next trip. There are always a lot of variables there too: traffic leaving and arriving town, long-term wind direction and strength, etc. The more you think about it, the more you can come up with to account for a small change.
Like I commented on Facebook to day though; with the fluctuation in fuel prices what they are, it's hit & miss whether you are actually saving any money (figuring you fill up when you are empty rather than when fuel prices are low.)
I've always thought it is ridiculous how people make such a big deal out of filling their tank when the price was a few pennies per gallon cheaper or even driving across town to a cheaper gas station... You COULD shut your air-conditioner off and save that gain in a day, probably.