# Thread: Accuracy of calculated milage

1. ## Accuracy of calculated milage

The past few times my wife has filled up our 06 Prius, she has performed the time honered step of calculating our milage by dividing the number of miles since we last filled up by the gallons of gas pumped at this filling. Her past two trys at this fall far short of the milage the "consumption" reading indicates. Whe she filled up yesterday, the car indicated we are getting about 51 mpg. However, we drove 380 miles since our last fill up and she put in 10 gallons.

She has suspected this for a few months and has diligently reset the milage indicator after each refill and doesn't always refill at the same station (thus ruling out pump problems).

Something doesn't add up. Are we missing something?

2.

3. ## Doing It right

You and/or wife are doing the correct math. A couple of things to re-emphasize:
1) The gas tank is actually a flexible gas bladder that creates a lot more back pressure on the gas pump than conventional cars. As a result, the variation of when a pump shuts off varies from gas pump to gas pump significantly on a Prius. This makes it hard to calibrate MPG on just one or two fill ups. Keep doing what you are doing to get more accurate long term results. I have been tracking MPG for years and still have a 3 to 5 MPG variation tank to tank.
2) I reset both the trip odometer B and the touchscreen MPG calculator to ensure that I have accurate mileage between fill ups. Every now and then I (or someone else) resets one or the other accidentally. The end result of an accidental reset is a big drop in calculated MPG.
3) If at all possible, exchange drivers for one tank. I know that on a Prius this may mean weeks, but individual driving characteristics really affect the MPG. I drive like an old man (because....I am an old man?) but yet my son gets better MPG.
4) If your tires are a different size than the originals, then this would make a difference. With an '06 I do not expect this, but I can only offer ideas.
5) The displayed MPG I have found to be about 2 MPG optimistic. I display about 47 MPG and long term calculation shows about 45 ('01 Prius). 38 actual vs 51 displayed is too much to ignore.

4. Originally Posted by FL_Prius_Driver
5) The displayed MPG I have found to be about 2 MPG optimistic. I display about 47 MPG and long term calculation shows about 45 ('01 Prius). 38 actual vs 51 displayed is too much to ignore.

My wife and I have an 07 and I have found the calculation to be just the opposite of yours, if it shows 47MPG my calculations are around 49MPG.