Calculating fuel economy

Posted on October 10, 2006 by Chris Lumens in car.

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.