But the meat of the car is the hybrid engine and fuel efficiency. So how does that do? The dashboard monitor tells me how I'm doing, usually somewhere between 40 and 50 miles per gallon. But I wanted to keep better track, so after my first year I decided to start recording my gas purchases; the following graph shows the record:
The dark blue line is the main one of interest - that's the average miles-per-gallon over 3 fills of the gas tank. The green line with considerably more variation is the single-fill average. I'm assuming that bounces up and down mainly because a fill isn't entirely well-defined, so sometimes the total in the tank after filling it is more or less than the average.
The main feature I notice in this is the seasonal variation: the MPG curve dips down to just a little over 40 MPG in mid-winter (January each year), and gets back up close to 50 somewhere in the middle of the year (though the peak seems to vary from June to October). I'm pretty sure this is due to the use of the car heater - when the heater's on, it forces the gasoline engine to turn on, even if the car otherwise doesn't need it to go anywhere. That defeats much of the purpose of the hybrid. But it does freeze here in winter, so heating's important...
There is also a very slight downward trend in my data. For the incomplete year of 2006, I was averaging 48.1 MPG (but that's missing most of the winter months). For 2007 the average over the full year was 46.4 MPG, for 2008 it was 45.4, and for 2009 it remained about steady at 45.5. For 2010 so far it's 43.9 MPG, but we have some warm months still to come to make up for the early part of the year, so perhaps it'll return to the 2008-2009 average. The downtrend could be just because the car's getting old, perhaps it doesn't slip through the air like it used to, or some mechanical parts are causing more friction than they should. It also could be due to changes in the pattern of my driving.
The light blue curve shows the average (over 3 fills) of the miles I drove per day at that time, and that shows a slight upward trend over the years. Part of that is from driving kids around more as they've been getting older; also November 2007 was the start of my regular gym attendance which has meant 2 short hops in the car most mornings.
The red curve is the price per gallon at the pump. Integrate all three curves and you can find out how much I've spent on gas over the past 3 and a half years: $3815. If I'd done all that driving in our minivan (18 mpg) the cost for gas would have been $9615, or an extra $5800. Of course if I didn't have the Prius I'd surely have picked a more efficient car than that - but it also wouldn't have been as fun to drive.
Anyway - I was chatting with a couple of old guys the other day who were reminiscing about their pickup trucks. Maybe I'll be that way about the Prius some day. Maybe I already am... it's definitely still a fun car to have 5 years later, whether or not I get to be smug about it :)