The first graph is a graph over time of the mileage per tank as reported by the car's built-in monitor.
There are a few data points missing as I'd forgotten to hit reset after filling up a few times, but I didn't miss often.
The second graph is both the monitor reading and the traditional method of calculating mileage: dividing the miles by the gallons at each fill-up.
As you can see, there's a little variation, but that seems to be more from errors in the traditional method. I'm ascribing the errors to the traditional method because there are known problems with that method. The averages of the two are close enough that it verifies that the monitor works well.
When averaging the averages, the traditional calculation comes out to 43.50 MPG over the entire period while the monitor comes to 44.41 MPG. It is possible that much of the difference is in the missing periods from the monitor information. If I use the traditional calculation method taking the current total miles on the car and dividing that by all the gallons I've put in the car, the average comes to 43.59.
You'll notice on the graphs that the troughs tend to come in winter and the peaks in summer. The car runs the AC on electric and doesn't affect the mileage much. In winter, the car runs the heat from engine heat and runs the gas engine more frequently reducing the mileage.
Since I also kept track of how much I paid for gas throughout this period, I was able to do a rough calculation on how much money I've saved over having kept the Pathfinder as my main vehicle. The Pathfinder got around 21MPG when I first got it, however now that it is at 263,000 miles and is 11 years old, it only gets around 16MPG. I made this determination over several tanks of gas just before I got the Prius. Most of it was highway driving, so that could actually be high.
I took the number of miles I drove for a tank of gas in the Prius and determined how many gallons I'd need (at 16mpg) to go the same distance in the Pathfinder. I then multiplied those gallons by the price per gallon at the time of that tank. I know there are problems there as I'd have to have filled up more often in the Pathfinder and prices may have varied, but it is unlikely they'd have been off by more than a few cents at a time.
So, how much have I saved on gas by driving the Prius instead of the Pathfinder over 2 and 1/2 years?
That is much more than I'd expected. This means if I drive the Prius for 10 years, it'll pay for itself.
Of course, the savings would be lower if I compare it to a new traditional car, but as the CAFE standard in 2006 was 27.5 the savings would be: $2,283.24
However, actual mileage of mid-size 2006 cars was 17.56 City, 25.34 Highway on average and the average of those two is 21.45MPG
Based on that the savings would have been: $4,029.72
Still pretty good either way.