Notes about Gas Mileage:

There are many factors that affect gas mileage in any car. However, there is a consensus among members of the Yahoo Prius forum that the Prius has some some unique factors that affect mileage. They are as follows:

• Gas tank bladder: To reduce gasoline vapor, the gas tank in the Prius has a bladder inside of it. This bladder changes size and elasticity with changes in temperature. This means that while the car's display may show 50MPG, the gas tank may take more or less gasoline since the last fill-up, resulting in a higher or lower MPG when actually doing the calculations yourself. As far as I'm concerned, the manual calculations provide the only useful mileage result since I want to know how much gas I have actually had to purchase (rather than how much I've used) over a given distance.

In my mileage logs on this site I always try to include the outside temperature at the time of fill-up. If temps have climbed since the last fill-up, mileage may be lower than anticipated, and in some cases a temperature dip will actually shrink the tank enough to create a more favorable MPG calculation.

Please also note that here in Minnesota we have wild fluctuations in temperature on a regular basis. In recent years we have literally had a foot of snow and 90 degree weather within the span of a week. So the temperature at fill-up does not always accurately reflect the typical driving conditions since the last gas station visit.

• Optimal temperature for emission equipment: In order to provide SULEV emissions, the Prius' emission equipment must be at optimal operating temperature. This means that the first 5 minutes or so of driving will usually yield lower MPG while the engine warms up the emission system. In the winter, the engine has to run even more to maintain this temperature, so cold temperatures will reduce mileage somewhat.
  
• A/C Use: The 2001-2003 Prius requires idling the engine to run the air conditioning compressor. With the climate control set to "Auto," this idling is kept to a minimum, but it will still lower mileage somewhat when in heavy traffic on a hot day. The 2004 Prius has an all-electric A/C system that will reduce the impact on mileage.
  
• Tire pressure: The 2001-2003 Prius has special low rolling-resistance tires. If these are not kept at the proper inflation pressure, mileage will be impacted. The Toyota inflation specs are 35psi front/33psi rear. Many people on the Prius forum recommend using 42 front/40 rear. I have never used these inflation numbers, but those who do swear it yields better MPG.
  
• Calculations are sensitive to small volume measurement errors: The Prius gets such great mileage that small fluctuations in the gas bladder coupled with small volume errors/differences at the gas pump can translate into large differences in mileage calculations. For example take a look at  the following data from my 2003 mileage log:

Here the odometer readings vary by only half a mile, and a tiny 0.077 gallon difference in gas purchased changes the mileage by 0.6 MPG! If the gas pump shuts off a bit early (or is inaccurate,) or if the tank's bladder volume is slightly different at fill-up, the calculated mileage will vary considerably!

In addition to these factors there are the usual ones that affect any car: how aggressive the driver is, driving with/against the wind, etc.

Please keep all these factors in mind when viewing the mileage data on this site!