Calculating fuel economy

Posted on October 10, 2006 by Chris Lumens in .

Some recent work that Ted has been doing about his fuel economy got me thinking about my own car (a 2005 Toyota Corolla). I have been recording every fuel purchase for the life of my car, including current odometer readings, price, gallons bought, and the mileage since the last fill up. So I have all the data. I even have some ideas about trends in my fuel mileage. I just haven’t sat down and analyzed it in any form. Today, however, I punched in all the data to OpenOffice and plotted some charts to see what’s going on.

The results since winter 2004 are shown on the following graph:

First let me explain what you’re looking at. The numbers along the bottom are months. Just ignore the slashes. Each point plotted is a fill up. The blue line is simply connecting the dots between all the fill ups. The red line is a five tank moving average, which is why it starts a little to the right on the graph. Here are the trends I see when looking at the moving average, since that is a little more telling:

I’m not sure of what sort of driving habits might be causing these trends. This year I have gone on a lot more long trips due to hiking, which may explain why summer 2006 is slightly higher than summer 2005. In the winter, I tend to take fewer long trips and more very short trips because of the road conditions. That might explain some of the sudden plummet in mileage. However, I have not investigated my driving habits enough to full explain it. Any ideas?

It also looks like I had better get used to another downturn in fuel economy in about a month, if previous patterns continue to hold.