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Is MPGe an evil unit? Why or why not?

Discussion in 'Energy, Environment, and Policy' started by EchoDelta, Feb 27, 2013.

  1. EchoDelta

    EchoDelta Member

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    I consider it is. While I wholeheartedly think it is important to have an efficiency metric on which to compare vehicles, I think MPGe is an abomination foisted on consumers by the ICE incumbents (car and oil companies) to confound efficiency discussions and to reduce perception of efficiency gained by making EVs for manufacturers.
    I am not debating whether distance or energy should be used as numerators or denominators (litres/100km etc is just the same in my mind)

    I am talking about reducing kWh to an equivalent "gallons/litres of gas" without incorporating into it the political, monetary, supply chain, and transport & distribution station costs of getting that gallon of oil into the car, and the environmental, carbon capture, etc costs that are externalized in that unit.

    It especially annoys me to see websites to buy/sell cars putting electric car MPGe under the same label as MPG, so you can compare a 40 mpg Honda Civic with a 89 mpg Tesla Model S. Then a website calculator asks how many miles you drive and with your zipcode calculates the $/gal and tells you the cost to drive the car over 5 years. And mentally, who will pay attention to the 'e' - consumers will mentally estimate a 90 MPGe car to be only 3 times as efficient as a 30 mpg car.

    I'm trying to see if there is something I missed on either side of the argument - do you consider MPGe to a useful unit for comparing car efficiency, or a distractive/destructive one - and why?
     
  2. markwj

    markwj Moderator, Asia Pacific

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    From my understanding, MPG and MPGe boil down to Mp$. That is certainly the intent.

    They do ignore the costs of the life-cycle of the fuel (well/mine to smoke), but so does society (today).

    So, are they meaningful when comparing the fuel running costs of two vehicles? Probably. Regional variances? No. Costs to society? No.

    That said, when comparing things, whoever gets to set the questions wins. If I was an ICE manufacturer doing a comparison, I'd list things like 'time to fill-up', 'range', etc. Even including the "G" in the MPG/MPGe comparison shows such bias.

    Consumers like comparison numbers, and these are not the worst I've seen.
     
  3. Doug_G

    Doug_G Lead Moderator

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    What it does is confuse people. Last year I showed my Roadster at the auto show in Toronto. The CAA, which had organized that exhibit area, put this sign in front of it with MPGe rating. Naturally I got all kinds of confused questions about whether it took gasoline. After a couple of days of that we tossed the sign behind a curtain. Problem solved.
     
  4. RDoc

    RDoc S85D

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    But doesn't MPGe greatly overstate the energy and CO2 emission advantages of electric vehicles and understate the energy independence advantage? It doesn't take into account the less than 100% efficiency of both electric generation and transmission nor the source of the energy. Something like 50% to 70% of the MPGe would likely be a more realistic number. It seems to me it depends a lot on what you are concerned about with energy use, CO2, raw energy, or energy security.
     
  5. dhrivnak

    dhrivnak Active Member

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    #5 dhrivnak, Feb 27, 2013
    Last edited: Feb 27, 2013
    Personally I have found the MPGe of the Roadster (119 MPGe) to be reasonably accurate. With some care I can drive 200 miles on about 50 KW of electricity or about $4. At $3.75/gal gas that is like getting 175 MPG in a gasoline car. So when I put the EPA sign showing 119MGe in the window at car show it gathers a lot of attention and give me ample opportunity to help educate the public. I use it to show people there is a BIG difference in efficiency. Both the EPA sticker and a cost comparison give similar ball park results.

    Tesla Roadster 2.5 snags official 119 MPGe rating

    And while I agree MPG number are not the best way to measure a vehicle's efficiency they are numbers that most people know and understand. We need to take change one step at a time.
     
  6. richkae

    richkae VIN587

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    Don't start with a bad approximation like MPGe and then add more fuzzy estimates and try to compare.

    If you want CO2 start with the CO2 produced from burning gasoline, but then add the CO2 produced from transporting it, evaporative losses, refining it, and extracting it.
    Since the energy used to transport it, refine it and extract it frequently comes from burning the dirtiest parts of crude oil - you will find that this is a lot of CO2 ( and other pollutants ).
    Then don't forget that most cars burn dirtier when they get old and out of tune, so that they are not nearly as clean as new.
    I would also argue that you should penalize the gasoline car with the CO2 that comes from burning the non-gasoline byproducts of refining - because if there were no demand for gasoline, we would not be burning all those other dirty secondary products, but would use cleaner sources instead.
    When you add it up, the CO2 from a gasoline car is a LOT more than just what comes out of its tailpipe.

    The CO2 from the power plant for an EV and its fuel chain are simple by comparison.
     
  7. dhrivnak

    dhrivnak Active Member

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    Not sure about your area but in NE Tennessee less than 5% of the people know or care about CO2 and for those who do I have a detailed analysis worked up. And in our mostly coal produced electricity mix the Roadster generates only 31% of the CO2 as a Corvette and is 78% that of the best in class Prius.

    But over half the people's eye's glaze over if you try to explain CO2. Then 45% feel it is their RIGHT to burn as much CO2 ad they D**M well please. So I normally take the tact that the "fuel" cost for the car is 15% the cost of gasoline or $1 to drive 50 miles, or I prefer to burn American made fuel not imported oil.

    I am afraid we still have a long way to go before the average Joe wants to reduce their carbon footprint.
     
  8. SCW-Greg

    SCW-Greg Active Member

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    Keep it simple, I say. How much does it cost to drive on a tank... or more precisely, how many $ to drive 300 miles? $8 maybe $10 for the S, vs. $50 to 60 or so dollars for an ICE.

    Coincidently, my Cadillac gets almost the exact same range per tank full (~270) with the commute I have. Not that its V8 is all that thrifty, but a very easy miles comparison.
     
  9. mulder1231

    mulder1231 Active Member

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    #9 mulder1231, Feb 27, 2013
    Last edited: Feb 27, 2013
    I find the European way of specifying fuel consumption easier to understand: # of liters of gasoline or diesel consumed per 100 km distance. For EVs the equivalent would then be # kWh of electricity consumed per 100 km distance.

    It makes it very easy to compare what it will cost you to drive the same distance, for example:

    Audi S6 consumes 6.35 gallons/100 miles (15.76 MPG)
    At $4 per gallon a 100 mile trip will cost you $25.40 (6.35 x $4)

    Tesla Model S consumes 38 kWh/100 miles (89 MPGe)
    At $0.14 per kWh a 100 mile trip will cost you $5.32 (38 x $0.14)

    Makes it easy to plug-in your local energy prices to estimate more realistic cost savings.
     
  10. stopcrazypp

    stopcrazypp Well-Known Member

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    MPGe is simply a unit that allows people to compare EVs to EVs using a similar rating system as an ICE car. It is almost worthless as a unit for comparison of EVs to ICE as it compares neither $/mile nor CO2/mile. The only use is that it tells you the wall to wheel efficiency (similar to how MPG tells you the ICE car's tank to wheel efficiency), which you can then use to calculate CO2 or wheel to wheel efficiency.
     
  11. EchoDelta

    EchoDelta Member

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    YOU understand this. But they all look like MPG-ish things to the average Joe. And websites allow that sort of comparison.

    I see a lot of 'a step at a time' comments; I guess my personal stance is not to compromise on things that help confuse the public.
    So when talking to an average Joe I speak of $/mile or equivalents ($/'tank' as in the example above); adding comments about the externalized costs of oil supply chain; and when talking to more educated folks I talk about CO2 impact and how buying RECs or solar helps reduce that etc (also good starter for solar conversations).

    Mind sharing? Does it include the operational aspect only; or also the capital expenses? Sorry for going offtopic.
     
  12. GSP

    GSP Member

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    Exactly.

    I find MPGe completely useless. I prefer to use kWh/100mi to compare efficiency, but I don't really care about this much.

    EV range and charging rates are much more interesting to compare EVs, as they have a much bigger impact on the car's usability.

    GSP
     
  13. Robert.Boston

    Robert.Boston Model S VIN P01536

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    While I generally agree with the foregoing arguments, I'll note that the existence of a MPGe rating on the Model S was the tool by which I was able to force Lincoln Motors to stop advertising the MKZ as the "most fuel-efficient luxury vehicle in America."
     
  14. mknox

    mknox Well-Known Member

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    I'll be the exception to the rule and state (as I have in other threads) that I prefer MPGe (MPG for ICE) and detest using the distance as the denominator (i.e. L /100 km) in these units.

    My rationale is that the unit of energy (be it a gallon of gas or a kWh of electricity) is something I understand, can "visualize" and I know what it costs. What I want to be able to quickly see is how far that gallon or kWh will take me. I never drive precisely 100 km and so how much fuel that would take is meaningless to me.

    MPGe lets you compare gas, diesel, electricity and other types of energy by bringing them all down to a "gallon equivalent" of energy. A bit awkward, and I would actually like to see miles per watt-hour or miles per kWh or something like that, but MPGe is okay.

    At the end of the day, I can do math, and for my purposes will always calculate back to distance per unit of energy if the data is not provided to me in that way.
     
  15. Al Sherman

    Al Sherman It's about THIS car.

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    I agree. However, if you are trying to show the average car consumer how much less expensive An EV is to operate there is always another calculation to get the ratio of cost/mile or cost per/"tank."
     
  16. montgom626

    montgom626 Active Member

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    MPGe is an evil unit

    MPGe an evil unit. Silly, meaningless and deceptive
     
  17. mknox

    mknox Well-Known Member

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    I feel exactly the same way about L/100km!

    - - - Updated - - -

    Absolutely. I have a little database app on my iPhone where I enter all of my fuel purchases. One of the fields I calculate is cost/mile. This varies, of course, with fuel cost and the efficiency I got out of that tank of gas.

    One of the things I do when I speak with people is point out how far the Model S goes on a "tank" and how much that "tank" costs. It's a real eye-opener. For instance, looking at my records, a couple of fill-ups ago, I went 257.7 miles on $72.50 of gasoline. That's roughly the EPA range of Model S, and if we say that would take 85 kWh of electricity, at my off-peak pricing it works out to $12.23. That's a $60 savings to drive the same distance! I drive a fair distance to work, and have to fill up roughly every 4 days. Saving $60 every 4 days is gonna be sweet when I get my car in a few weeks!
     
  18. brianman

    brianman Burrito Founder

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    @mknox - I agree with you on the distance in the numerator. Can we agree on a currency number in the denominator (rather than a fuel or energy quantity)?
     
  19. Robert.Boston

    Robert.Boston Model S VIN P01536

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    The problem with a $/mile or €/km metric is that it isn't constant--by geography or time. In places like California, two neighbors may have different $/mile because of the tiered rate classes.
     
  20. RDoc

    RDoc S85D

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    I agree with mknox, distance in the numerator simply because at least in N America that's what people are used to. I'd put kWh in the denominator as that's what people pay for, so Miles/kWh. As I said before, I think the comparison to gallons is very misleading because there are so many dimensions over which the two energy sources aren't the same. The problem with cost, as Robert pointed out, is that it's too variable.
     

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