Monday, March 30, 2009

Switching to the Prius

I have had my current Prius since late in 2003. Since then, my overall MPG figures are like this:

Overall Average MPG (calc) 48.47
Overall Average MPG (Screen) 49.14

Winter Average MPG (calculated) 47.10
Winter Average MPG (Screen) 47.80

Best Tank (calculated) 65.63
Best Tank (Screen) 57.60

Most Recent MPG (calc) 43.47
Most Recent MPG (Screen) 44.00

Summer Average MPG (calculated) 50.76
Summer Average MPG (Screen) 51.38

Worst Tank (calculated) 31.88
Worst tank (Screen) 41.40

Average MPG Actual

Average MPG Screen

Calculated vs. Screen

Percent Difference

Note, this is for all types of driving in all seasons. Also for those who may not know, the Prius currently has a fuel tank bladder (for emissions control) that makes the tank size vary from fill up to fill up. That is why I am tracking both the standard miles travelled since last fill up, divided by fill up gallons as you would do to calculate your own MPG. And I am tracking the Prius’ own on screen prediction for each fill up. I also compare the differences, and as you see over time the difference between methods is not very much. How does the Prius figure its own MPG? As I recall, it monitors the fuel flow at one fuel injector, using that to calculate the total fuel consumed, and then divides by miles traveled. The result of the comparison is that individual tanks vary greatly depending on the temperature of the bladder. But, over time this will even out. You also see differences by winter or summer driving. For my purposes, I am calling the warmer months summer, and the colder ones winter. This is for the sake of simplicity. Also, for the statistically minded people, I am using a weighted average to calculate the overall average from the Prius interior display of MPG. It should also be noted that the Prius, along with all other cars, recently underwent a change in how the estimated MPG for the window sticker of a new car is calculated. When I bought mine, the estimated MPG was as listed under “Old MPG estimates”. At the time, my overall MPG was just at 50 MPG, missing the EPA figures somewhat. Now, with the new predictions as listed, I would make that combined MPG with room to spare, even if my current overall listing is dropping to 49.14 as stated above. Why is my MPG dropping? I have different tires now than when I started, and evidently the MPG is a bit lower than the standard issue. Also, I am making shorter trips now than initially. More on that later. . .

Old MPG estimates
60 City
51 Highway
55 Combined

New MPG estimates
48 City
45 Highway
46 Combined

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