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*Your* MPGe

Discussion in 'Model S' started by eelton, Jan 10, 2013.

  1. eelton

    eelton Member

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    #1 eelton, Jan 10, 2013
    Last edited: Jan 13, 2013
    The government rates the Model S at 89 MPGe. As gregincal pointed out to me, this is based on the energy content of gasoline (see his post #6 below). However, I think it is useful to calculate another type of [cost-adjusted] MPGe which incorporates the local cost of electricity and gasoline, given the widely varying electricity costs--and also gasoline costs--around the country. This allows a cost comparison to one's ICE cars.

    So, I calculated my cost-adjusted MPGe, considering that I pay 5.79 cents per kWh, and my cost for gasoline is $3.99 per gallon (I used the cost for premium, since that's what all of my cars take). I'm helped here by the relatively low electricity and relatively high gasoline costs in Chicago, and also my use of an alternative electricity supplier. For me, that gives an MPGe of 181*.


    I made a simple spreadsheet to calculate this, which is here. I used the formula MPGe = (miles per kWh)*(cost of gasoline per gallon)/(cost of electricity per kWh). [Miles per kWh is 100/(kWh/100 miles)]. The Model S with the 85kWh battery pack is rated at 38 kWh per 100 miles.


    So, what is your cost-adjusted MPGe?




    *I posted this later on, but adding in all of the transmission fees, distribution fees, and taxes on my electric bill brings this down to a less-lofty 114 MPGe. Still not bad, though.
     
  2. efusco

    efusco Moderator - Model S & X forums

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    Just got jibberish when I tried to open your link, but well done and thanks for sharing.
     
  3. eelton

    eelton Member

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    #3 eelton, Jan 10, 2013
    Last edited: Jan 10, 2013
    Not sure why the link didn't work, but here is the file.

    [A bit off topic, but the problem is apparently Internet Explorer's handling of links to Office files. It works in Chrome, Firefox, and Opera--at least for me.]
     

    Attached Files:

  4. efusco

    efusco Moderator - Model S & X forums

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    Thanks, that worked. At 34kW/100 miles, $3.91/gallon and $0.11/kW my MPGe is 105
     
  5. kvietor

    kvietor Model S S280 VIN 168

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    Ive been averaging 305 w/mi over the last 4000 + miles. Gas around here is currently @ 3.40. So about 101MPGe.
     
  6. gregincal

    gregincal Active Member

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    The governments conversion of electricity to miles per gallon has nothing to do with the price of electricity. It has to do with the energy content of gasoline. 1 gallon of gasoline contains 115,000 Btu and 1 kWh contains 3,412 Btu, so 33.7kWh = 1 gallon of gasoline. Your way is a good way to demonstrate the cost savings of an electric vehicle due to the cheaper cost of electricity, but the government's method just measures the efficiency of the motor without depending on prices.
     
  7. kvietor

    kvietor Model S S280 VIN 168

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    So using those numbers, how do they get 89 MPGe on the 85 kWh battery?
     
  8. aviators99

    aviators99 Model S - R140

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    Isn't it actually rated at 32 kWh per 100 miles (if the rating is 265 miles per 85 kWh)?
     
  9. brianman

    brianman Burrito Founder

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    Original link works fine for me on Win7, IE9, Excel 2010.
     
  10. eelton

    eelton Member

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    Thanks. You're right. The government sticker does use an average electricity rate of 12 cents per kWh in calculating the annual fuel cost, but not the MPGe.

    - - - Updated - - -

    The EPA rating is 38 kWh/100 miles for the 85kWh battery pack and 35 kWh/100 miles for the 60kWh battery.

    See here. (click on the midsize and large car tab).
     
  11. aviators99

    aviators99 Model S - R140

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    Then how do they get 265 rated miles from 85kWh?
     
  12. ElSupreme

    ElSupreme Model S 03182

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    The MPGe number takes into account charging losses. It is power 'from the wall' in the measurement. So their total power over a full battery is more than 85kWh. While the range number is solely based on the usable part of the battery.

    This is why the two numbers don't add up.
     
  13. Plug Me In

    Plug Me In Member

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    Do you think, as alternate fuel vehicles (not just EV's) become more prevalent that EPA will switch to a cost per mile standard? To me, simpler to calculate and provides a more clear comparison between vehicles.
     
  14. ElSupreme

    ElSupreme Model S 03182

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    Fuel prices are too time/location/regionally variable. And electricity costs are plan/time/regionally variable also. So I doubt it. I don't know if kW/100 miles might become more common than MPGe. I think this is more the shift that will happen.

    And I personally am hoping for kW/100km to become the standard. Because the Metric system really is much better.

    The EPA sticker already has an 'estimated annual fuel cost' line on it. This is about as good as you will get I think.

    That is until cars get ePaper internet linked stickers on the windows to update on the fly.
     
  15. aviators99

    aviators99 Model S - R140

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    That makes sense. It also means that it is not the right number to use for eelton's formula. 32 (if you want to use EPA) is, or 1/10 what your actual average Wh/mi is. This number happens to *be* 32 for me, at the moment. On my recent road trip, it was 28.
     
  16. mknox

    mknox Well-Known Member

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    double-facepalm1.jpg
     
  17. ElSupreme

    ElSupreme Model S 03182

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    You try doing heat or energy calculations with the standard system? It sucks!
     
  18. mknox

    mknox Well-Known Member

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    Sorry... you know how some people are real Democrat/Liberal or Repubican/Conservative zelots? That's me with the metric system :wink:
     
  19. ElSupreme

    ElSupreme Model S 03182

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    Honestly length, temperature, weight the US/Imperial/Standard system isn't too bad to deal with. I would say the metric system is easier, but it isn't that hard using miles, feet, and inches. But when you start putting them together to get more complex units the whole standardized thing falls apart. Metric keeps together much better.

    Besides the metric system has its own problems. This Kilogram Has A Weight-Loss Problem : NPR
     
  20. jomo25

    jomo25 P4398

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    And using these two numbers, we can determine what the "rated" charging loss/efficiency of the 85kWh variant Model S.
     

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