I calculate mileage as total miles traveled divided by total gas pumped for a particular time span. The Prius has a flexible gas tank, which shrinks as the tank is emptied. It also changes size based on temperature and air pressure, so "full" on one day is not necessarily the same number of gallons as "full" on a different day. Therefore, it is not meaningful to calculate tank mileages.
Lifetime Mileage: 42.45 mpg (as of 6/23/09)
Seasonal Summaries:
| MPG | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 |
| Winter | - | 39.83 | 36.05 | 38.13 | 39.57 | 39.96 | 36.28 | 37.07 |
| Spring | - | 43.41 | 42.95 | 44.68 | 43.05 | 45.80 | 42.86 | 42.26 |
| Summer | 43.66 | 45.39 | 46.55 | 42.58 | 44.16 | 48.73 | 44.20 | - |
| Autumn | 40.68 | 42.16 | 43.43 | 41.60 | 44.19 | 42.77 | 42.37 | - |
| Annual Summary | 41.44 | 42.29 | 42.71 | 42.05 | 42.82 | 44.60 | 41.90 | - |
Note 1: In the Spring and Winter of 2003, I taped my front grill at the advice of the Prius community; however, I did not tape it in Spring and Winter of 2004 to check for an impact. There was a noticeable effect of several mpg between the two years, so I taped it again in later years and was pleased to see a higher mileage again!
Note 2: The mpg for Summer and Autumn increased the first two years, probably due to continued break-in of the car.
Note 3: The mpg for Summer and Autumn dropped dramatically in 2005, because most of our long trips from that time forward include carrying camping gear on the roof rack and bikes on a trunk-mounted rack. These racks significantly impact drag. The effect was not as bad in later years, because we now try to drive on lower-speed-limit highways when carrying racks on long trips.
Note 4: In Autumn 2006 I started using a few hypermiling techniques: holding a constant throttle at highway speeds and using pulse-and-glide at city speeds. In general, the mpg since then has been higher. However, I'm not the only driver of the vehicle.
Note 5: Winter/Spring 2008 was one of the coldest and snowiest on record, which had a noticeable negative impact on mileage.