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2008 vs. 2009 Fuel Economy MPG

Discussion in 'Gen II Prius Fuel Economy' started by kocho, Mar 29, 2012.

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  1. kocho

    kocho Member

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    Looking at Fuelly, Toyota Prius MPG Reports | Fuelly , the 2009 Gen II seems to be 5 mpg better than the rest of the Gen II.

    Do you know if there is something to this other than variation in users who bought an '09 vs. the '04-'08 versions? There are plenty of cars in each year, so it is likely not due to one or two very frugal owners pushing the total of a small group up...

    Yesterday I just bought a used '08 so the question is a bit academincal at this point, but I'm still curious if someone has a definitive answer.
  2. F8L

    F8L Protecting Habitat & AG Lands

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    There is no difference in mpg capability of the different years. The driver and location is what is making the difference.
  3. uart

    uart Senior Member

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    There is no difference in the 09 model that would give it a 5 mpg improvement so yeah it looks like it's just a statistical anomaly.
  4. Danny Hamilton

    Danny Hamilton Active Member

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    I suspect that this guy is skewing the numbers a bit with his entry of 394.7 MPG in the 2009 model year:

    Prius III Lounge (Toyota Prius) | Fuelly

    I recomputed the 2009 numbers without his entry and I get 45.2 which is only 0.1 MPG better than 2008.
    2 people like this.
  5. uart

    uart Senior Member

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    Thanks for finding that anomaly Danny. :)

    I was just analyzing the data and based on a standard deviation of 6 MPG (per car) the probability of the 2009 data being that much higher due to purely random chance was literally one in one hundred trillion! (no joke or exaggeration). So yeah, it was obvious that something weird was going on there.
  6. Danny Hamilton

    Danny Hamilton Active Member

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    Also note that as far as I can tell, the site seems to be averaging the averages. In other words they seem to be just adding up the lifetime MPG of each car in a given model year and dividing by the number of cars in that model year. Generally when you average averages like that you tend to get skewed and even inaccurate numbers.

    It would be much more accurate to add up the total miles driven by all the cars in a given model year, and then divide by the total miles driven by those cars.

    Example:
    Vehicle 1 = 20 tanks of gas: 7200 miles: 45.0 MPG
    Vehicle 2 = 60 tanks of gas: 21600 miles: 45.7 MPG
    Vehicle 3 = 10 tanks of gas: 1200 miles: 30 MPG

    Total model year miles = 30,000
    Total model year fuel = 672.65 gallons

    30000 / 672.65 = 44.6 model year MPG

    (45.0 + 45.7 + 30) / 3 = 40.23 model year MPG???
  7. cwerdna

    cwerdna Senior Member

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    Yep.

    However, 07-09 Priuses could be a bit lower than previous ones due to the Touring model that started in 07. There's a slight mileage hit due to the larger wheels and tires.
  8. kocho

    kocho Member

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    Thank you all for the notes! The 300+mpg certainly can skew the results. Driving around town for the first 100 miles or so (we just bought the Prius) I'm hovering around the 48-50mpg on the display so that would likely end-up around 45-48 real miles. Will see how it goes when I do the first fill-up...

    I do have non LRR tires so my mileage might be a couple mpg lower than ideal, but the ride and handling are much better on the 195/60-15 Michelin Primacy MXV4 tires...

    A little disappointed though in the '08 - when I drove my friend's '10 I was hovering around 60mpg and on mine it seems it is around 50 with a fairly light foot on the gas... That's barely a couple of MPG more than what I was getting from my '02 while I still had it... Anyway, that's my wife's car now so let's see what she gets out of it. If she beats the 40mpg she gets in our Honda Insight she'll be happy -;)

    Thanks again for the info!
  9. uart

    uart Senior Member

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    It can often be a bit misleading to judge the FE like that. Much better to compare the average over a full tank or more.

    The Primacy MXV4 tires aren't too bad for rolling resistance, but you might pick up another 2 to 3 MPG with the lowest rolling resistance tires available. I picked up about an extra 2 to 2.5 MPG when I switched over to my current Bridgestone Ecopia EP 100's.
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