It's been almost 3 years since my last post about my 2005 Toyota Prius so I was thinking another update wouldn't hurt. The car now has 144,000 miles on it, and still runs fine as far as my daily use is concerned. Maintenance costs have not been high at all; I bring it to the dealer two to three times a year for oil changes and other routine maintenance. I haven't been quite as diligent as I was for the middle years there tracking receipts for gas. In particular there was a big gap with lost receipts in the summer of 2013 (reason lost in the mists of time), and a smaller gap in November & December 2015 when my son was driving it a lot and didn't give me all the receipts for gas he bought. But there's enough coverage that I can still see pretty accurately how it is performing with regards to mileage. First the full-year mileage averages (taken by dividing miles driven in the year by the number of gallons of gas purchased):
Clearly while there is some up-and-down movement, the trend has continued downwards. The graph of 3-fill-averaged mpg also shows the clear downward trend with what looks like clear seasonal variation. In the following I've fitted the observed mileage to a model consisting of a linear downward trend plus a sinusoid with 1-year period, and the fit looks pretty good:
The fitted sinusoid peaks in mid-August, and has its minimum in mid-February, which correlates with my own general observations that summer mileage tends to be much better than in the winter. The actual observed mileage numbers seem to spike even more sharply in summer and winter than the sinusoidal pattern (many points higher than the fit in the mid-summer, and lower in mid-winter). I don't think I have a good explanation for the pattern other than the effect of the outside temperature directly on the performance of the vehicle - air conditioning in mid-summer is surely as much a drain on performance as heating in mid-winter.
In addition to the seasonal variation of course there is the general decline in mpg level. The fit gives this as -0.002 mpg per day, or -0.73 mpg per year, which agrees generally with the total-year averages I posted above. Looking around the internet, possible explanations are gradual declining performance of the hybrid battery itself (it needs a little more energy to fully recharge as it ages), or possibly decline in some other components of the vehicle such as the catalytic converter (the engine may need to run for longer to get it hot enough to work properly). What was originally close to a 50 mpg vehicle is, after 11 years, definitely below 42 mpg averaged over the year (of course on occasion I still see 50+ mpg for short periods of time). It's still good gas mileage, but it's not as astonishingly good as it once was. Another 10 years and it likely won't be any better than the old Saturn I had years ago. Unless there's some maintenance effort that would reverse the decline - I wonder if replacing the battery would be a reasonable option in a few years? Batteries do seem to be cheaper these days!