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:
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:
30.84 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:
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.