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Diesel mpg underrated on the window sticker

Discussion in 'Diesels' started by Trollbait, Sep 19, 2013.

  1. Trollbait

    Trollbait It's a D&D thing

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    Many diesel drivers know that the window sticker EPA figures under report what the vehicle can get. Apparently, the EPA also knows this also.

    http://www.epa.gov/carlabel/documents/420r06017.pdf

    The PDF is on the study the EPA did for the label and testing changes for fuel economy testing in 2008. There is a nice little table on page 16, but I don't how to neatly cut and paste it to a post. So here is a summary of the data.
    The data comes from the Oak Ridge National laboratory Your MPG program. It is a collection of user reported data. It appears to have been rolled into the fueleconomy.gov site as the user mpg number under car entries. The MPG-based label where diesels did 18% better became the mathematically adjusted one used in 2008.

    As with other user reported sites, like Fuelly, there is selection bias. Only the people interested in mpg are reporting, and they are likely driving more efficiently and/or have a more efficient model. This would be of all the cars reported in the data, not just the diesels. So while the average diesel driver may not be getting that 18% difference, they are likely getter mpg in comparison to the label than gasoline drivers. An assumption, but a reasonable one.

    Not diesel specific, but of some interest:
    MOVES is a modeling system of vehicle emissions. MOVES (Motor Vehicle Emission Simulator) | Modeling and Inventories | US EPA

    So the EPA uses 43/57 city/highway split for one program, and this may be what actual drivers experience, but uses 55/45 for the combined number on the window sticker and website. I'm not sure how the "embedded in the 5-cycle formula" works into this. It may not even hold true today and have changed seeing how this paper was written before implementation.

    Edit: adding a current link for the paper. Document Display | NEPIS | US EPA
     
    #1 Trollbait, Sep 19, 2013
    Last edited: Feb 24, 2017
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  2. GrumpyCabbie

    GrumpyCabbie Senior Member

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    Living in a country where 50% of cars are diesel, I can assure you they don't get close to hybrids (for similar size, weight etc).
     
    #2 GrumpyCabbie, Sep 20, 2013
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  3. austingreen

    austingreen Senior Member

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    I posted information on a paper on this earlier. The 5 cycle testing is used to mathematically create the two number city/highway numbers then the 55/45 percentage of miles to create the combined figures. Companies can and mainly do use a 5 cycle derived version, where only the 2 cycle test is done, then they mathematically derive the figures as if all 5 cycles were tested. The epa in 2006 used numbers from 5 cycle testing, combined derived figures, and used it with real world numbers to create fudge factors for the tests. The prius was one of the vehicles in this test to create the fudge factors for both deriving city/highway and going from 2 cycle to derived 5 cycle.

    The 43/57 cycle is what the average american typically drives, but using 55/45 is how american drivers perceive themselves to drive, the fudge factors to get combined were probably in correct. The epa uses national averages for how much air conditioning or cold start to include in the calculations. YMMV.
     
    #3 austingreen, Sep 20, 2013
  4. Trollbait

    Trollbait It's a D&D thing

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    It isn't a comparison between drive types, but a comparison of the types to their EPA window sticker numbers. Because diesels do better than their labels doesn't imply that they do as well as hybrids.
    The people submitting their data to Oak Ridge were the ones stating a 43/57 split in their drives on average. At least those that filled in that section did.
     
    #4 Trollbait, Sep 20, 2013
  5. austingreen

    austingreen Senior Member

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    I may have misremembered that from the paper. I could not find the paper, but here are some slides including the 5 cycle calculation percentages for gen III prius that were put out before the paper was published.

    http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2012/veh_sys_sim/vss065_lohsebusch_2012_o.pdf
     
    #5 austingreen, Sep 20, 2013
  6. xraydoug

    xraydoug Active Member

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    So I am intrested what diesel gets a better fuelly mpg than the epa combined mpg rating? I know my prius c fuelly averate is 2% higher than epa combined listed on window.

    What diesel gets a better mpg that is of equilivnet size to a hybrid. then consider fuel is 25% more expensive than reg gas in my town.
     
    #6 xraydoug, Oct 3, 2013
  7. Trollbait

    Trollbait It's a D&D thing

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    The average of the two 328ds on fuelly is 14% higher than combined. The Golf and Jetta are 14% to 15%. The Passat is 17% to 18% higher.

    The Passat has about the same cabin space as the Hyundai Sonata hybrid, with a lot more trunk space. The numbers at fuelly have the Passat doing better.
    Compare Side-by-Side

    The price difference here is only around 15%. The cost of USLD is a hurdle for many in getting a diesel. But not everyone wants a hybrid, or even have one as an option in the car segment they are looking at, or they actually do drive most of the time on the highway. Choice is good.
     
    #7 Trollbait, Oct 3, 2013
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  8. xraydoug

    xraydoug Active Member

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    I am all for choice. my experiance with diesel is it cost me more to buy and my mpg was good but not great. and I drove the rabbit diesel to get the very best mpg and still got about 40 mpg. on prius c I am getting over 60 mpg. it has 5 star crash rate and AC cruse all the nice things. my cost per mile for fuel is about 60% of driving my rabbit diesel. and this car feels safe on the freeway next to larg trucks it dosent get blown all over the road like my rabbit.:)
     
    #8 xraydoug, Oct 3, 2013
  9. Trollbait

    Trollbait It's a D&D thing

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    What year was the Rabbit? IFAIK, the only diesel ones were from the '80s. It may not have been geared for today's highway speeds for best efficient. I do know they didn't have a turbo, which improves a diesels efficiency and performance.

    Under the revised EPA numbers, the 4-spd manual had a 38mpg combined, the 5-spd got 36, and the 3-spd automatic got 29.
    Compare Side-by-Side
     
    #9 Trollbait, Oct 4, 2013
  10. GrumpyCabbie

    GrumpyCabbie Senior Member

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    The latest Golf diesels are actually very nice, very quick and economical so I'm not sure which you're refering to Xraydoug.
     
    #10 GrumpyCabbie, Oct 4, 2013
  11. xraydoug

    xraydoug Active Member

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    It was an 1980, it had a newer engine and transaxel from an 81 it was a 5 speed. it started right up even when cold out. it was a good car for around town. but sure was not as cheap to drive. going from 40 mpg diesel to 60 mpg gas is huge. I don't even think that the epa gave mpg est for diesel cars? It dosent matter my cost per mile to drive is now only 60% of what I was paying for the diesel rabbit.

    The prius did cost me about 17k more. than I sold the rabbit for but it is a new car and I can jump in and drive without elecrical issues the rabbit had. :confused: also driving any distance on the freeway with the old car was a fearfull thing:sick: the car would drift when a gust of wind hit it from big trucks ect.. and yes it had a recent front end alignement replaceing all the worn parts. good breaks. all the maintenance was good.
     
    #11 xraydoug, Oct 4, 2013
  12. Trollbait

    Trollbait It's a D&D thing

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    I learned to drive manual in my friend's 1980 diesel Rabbit.
    It had the electrical issues.
    It is comparatively smaller, and likely lighter, than modern cars in its class.
    I'm sure there were mpg numbers, but the fueleconomy.gov site only goes back to 1984.The original numbers for the 84 were 40/44 city/hwy. The adjusted are 33/40.
    Depending on your driving mix(I'm guessing mostly city with your c numbers), 40mpg was good for the car.
     
    #12 Trollbait, Oct 4, 2013
  13. xraydoug

    xraydoug Active Member

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    I think the car was doing exactly what is was built to do. It is just outperformed in everyway by my c(y)
     
    #13 xraydoug, Oct 4, 2013
  14. jgilliam1955

    jgilliam1955 Sometime your just gotta cry! 2013 Prius 4.

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    Folks should check Fuelly.com to see the real life numbers. They are the combined numbers for MPG. The Prius 2013 is 48 MPG combined. The Jetta TDI 2013 is 42 MPG combined.
     
    #14 jgilliam1955, Nov 9, 2013
  15. Trollbait

    Trollbait It's a D&D thing

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    Which just illustrates what the study says about the window sticker numbers underrating a diesel. The Prius is rated 50mpg. So the real world number is 4.2% lower than EPA. Regardless of transmission type, the Jetta is 34mpg combined on the sticker. Which means the real world numbers are 33% higher than the published EPA rating.

    So, with all the bother the gen2 Prius and original Insight caused earlier, and the Ford hybrids recently, leading to testing and reporting changes in the recent past and possibly in the future, why don't we also adjust the procedures to more accurately reflect what diesels get in the real world?
     
    #15 Trollbait, Nov 9, 2013
  16. Sergiospl

    Sergiospl Senior Member

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    Why not go for an even 50% adjustment, 'cause diesel's real world $ numbers at the pump: 25% more; http://www.floridastategasprices.com/RaceTrac_Gas_Stations/Port_Richey/143503/index.aspx
     
    #16 Sergiospl, Nov 15, 2013
  17. wjtracy

    wjtracy Senior Member

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    I only care about EPA MPG when it matters.
    For example, Cash-4-Clunkers I was forced to buy a Dodge Caravan (20 MPG) because Siena was not allowed (only 19 MPG). In reality my van is lucky to get 17 MPG and the Sienna probably gets closer to the 20 MPG.

    So as long as they don't write a law around EPA MPG we are OK. But we do have the new 54.5 CAFE requirements coming out, I hope in some way the Fuelly results can be used by manufacturers to apply for better CAFE MPG if that's what people are actually getting.
     
    #17 wjtracy, Nov 15, 2013
  18. Trollbait

    Trollbait It's a D&D thing

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    #18 Trollbait, Nov 16, 2013
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