The correct way to compare efficiency.

Discussion in 'Gen 2 Prius Fuel Economy' started by Alric, Jun 20, 2008.

  1. Alric

    Alric New Member

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    Gallons Per Mile Makes More Sense

    It turns out that comparing MPG gives a false idea when comparing the fuel efficiency of cars. As I suspected fuel efficiency does NOT decrease evenly as MPGs decrease. Cars with low MPG are drastically much less efficient than cars with average MPG.

    YouTube - The MPG Illusion

    [​IMG]
     
  2. Popeye

    Popeye New Member

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    In general I agree, l/100k or g/100miles might make more "intuitive" sense to people, but, when you get right down to it, it's the same data. 1/10 = 0.1, 1/20 = 0.05, 1/25 = 0.04, 1/50 = 0.02. What the study really shows is how poor the math and logic skills of the average American consumer are.

    It also shows that the way in which data is presented and questions are asked can change the results. In this case, the proponents want to show that presenting the data in the inverse form provides a more "intuitive" decision.

    How about asking the question "which would be better, replacing a gas-hog vehicle with one that gets twice the mileage, or replacing a relatively efficient vehicle with one that gets twice the mileage"? I bet the results would be different. Using various trigger words (like "gas-hog") can have a profound effect on the result. Ask the political pollsters, they're pretty familiar with that manipulation. The study does not appear to take into account the way in which the rest of the data/question was presented.

    They also had their "consumers" make the assumption that each of the "old" vehicles is driven the same amount every year. That's probably not realistic. In order to make the decision they are requesting, a consumer should sit down and analyze the number of miles driven in each vehicle, the uses of the vehicle, the needs of the family, the fuel efficiency, and a host of other factors. If the consumer actually has the education to do that analysis, then the units in which the information are presented become immaterial. If I am considering a vehicle that gets 20 mpg, and I expect to drive it 10,000 miles per year, it's pretty easy to do 10000/20 = 500 gallons per year. If I know instead that the vehicle gets 5 gallons/100 miles then it's 10000/100 * 5 = 500 gallons per year. I'd say that it's a wash which is easier for me to do. The harder part of the analysis, which can't be presented on the window sticker, is figuring out what I'm going to use the vehicle for, how much it's going to get driven each year, what features it needs to have, what features I'd like it to have, etc. After all of that, figuring out how much gas each of my options uses is pretty easy. And I don't have to do any math with either approach, I just need to know if I'm looking for the lowest number (g/m) or the highest (mpg) amongst all of the vehicles that meet my other criteria.

    If instead of a reasoned analysis the decision is based on "feeling" or "intuition" then the data presentation won't make much of a difference. And thus, where we find ourselves today....
     
  3. V8Cobrakid

    V8Cobrakid Green Handyman

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    ... cool link. it's kinda childs play though. i mean.. do you really need a bunch of wanna be college grads to figure out that it takes distance to figure out your fuel savings?

    I try to ask people what their mileage is.. or.. ask them how much tanks cost.. honestly.. it takes a price and mileage count of a tank to teach people. most people don't know how many miles their current vehicle goes on a tank... or 100 miles... the most common answer is " how far is 100 miles".. or.. i get epa figures of x and x right?

    soo.. a number of x per x miles does nothing for people... because there is nothing that calculates for climate indifferences.. or short trip commutes. not to mention.. no source off hand that says the compairisons. that's what we have the EPA figures for.
     
  4. miscrms

    miscrms Plug Envious Member

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    As far as I know pretty much every other country in the world calculates it this way (litres/100km). I think it makes a lot more sense, because usually the amount of travel we have to do is fixed. We need to get from point A to point B, so how much fuel is it going to take? Obviously you can figure it out from MPG, but I don't think most people do so. Its just one more factor isolating people from having a sense of how much fuel they actually use.

    Here's an example. Chevy advertises the Tahoe Hybrid as 50% more efficient! Joe Consumer would reasonably expect then that his fuel bill will be reduced by 50% or cut in half, sounds great!. Of course Chevy is calculating based on city MPGs. If you calculate based on combined MPGs, the Tahoe Hybrid is 31.25% more efficient (16 vs 21). But now if we flip that over and look at the amount of fuel consumed, the Tahoe hybrid uses 23.7% less fuel (1.19 gal/25miles vs. 1.56 gal/25 miles) than the non-hybrid. 23.7% isn't terrible, but pretty disappointing if you were expecting half.

    Rob
     
  5. timwalsh300

    timwalsh300 Member

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    He makes a good point about driving up the hill at 10 mpg and down the hill at 100+ mpg.

    50 mi @ 10 mpg = 5 g
    50 mi @ 100 mpg = 0.5 g

    100 mi / 5.5 g = ~18 mpg

    Notice that changing 100 mpg to infinity on the way down does very little. It only increases our total to 20 mpg.

    Now consider...

    50 mi @ 25 mpg = 2 g
    50 mi @ 100 mpg = 0.5 g

    100 mi / 2.5 g = 40 mpg

    We did better (MUCH better) by multiplying our fuel economy going up the hill by 2 than we did by multiplying our fuel economy going down the hill by infinity.

    It re-emphasizes the importance of attacking a hill correctly. Accelerate before you reach the hill where it only costs 25 mpg and then coast to the top. Don't accelerate on the hill where it costs 10 mpg. The difference is significant.

    Tim
     
  6. donee

    donee New Member

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    Hi All,

    One can make any numbers up about going up and down a hill. Unless the numbers are from real operation from a real vehicle, the results are useless. I did a glide down a 2 mile hill on Friday at infinite mpg and 40 mph. I did not record my up hill consumption however. But, one can slow down to 15 mph up the hill, and get like 45 mpg for the uphill. If the up and down are equal, then one is getting 90 mpg with an average speed of 27.5 mph.

    If it takes two PHD buisness profs to realise the mathematics of division, My gawd what is the level of education at that school? But the fact is also that one gets 25 free miles in the 50 mpg vehicle, that one does not get in the 25 mpg vehicle. Seems to me this is some kinda marketing ploy to get people to consider something like a Malibu hybrid.

    Also on U-tube this week there are some other fluff FUD marketing propaganda pieces. Like the supposed PBS show (I have never heard before) that interviews Art Spinella but none of the Argonne, RMI or PacInst scientists that have thoroughly debunkend his psuedo-science. Art comments on Prius complexity and fails to mention the Hummer 4wd drive train complexity. You know they have all sorts of fancy clutching, and positraking types of things so that a single traction wheel keeps the Hummers moving on rough terrain.

    Or the RPMFreeaks episode that promises to give you Prius hypermiling tips, only to comment on 30 pounds of nickel in the Prius battery, when they show a PHEV battery which is 3 times the size of the Prius battery, and takes up the whole underfloor portion of the trunk in their video.

    The marketeers are alive and well, and out there spreading lies again!!!
     
  7. DeadPhish

    DeadPhish Senior Member

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    Here are the key differences in the two measurment methods. Obviously they are just recipricals so there's no mystery but...

    one system, the current one - mpg, measures how far you can go on one gallon of gas. In our wide open spaces this used to make sense. 'How many gallons do I have to put in to get from NY to Boston?' It's almost become a bit of a contest "I can go 50 mi on one gallon vs your 18 mi on one gallon, Hah!'

    Nowhere in either of these two statements is there any sense of responsibility. It's all about some ephemeral distance contest. "D*** measuring' comes to mind.

    'How much fuel did you use [consume, waste, burn up] going from NY to Boston?' I don't know ( or care ), I just drove it.

    How much does your SUV consume each day?
    How much does your Prius consume each day?
    How much do you use each week commuting?

    Who cares what the distance is being driven now, today it's about consumption. See my sig below.. I use 2.1 gal / 100 mi driven ( GPC )

    It's easier IMO to measure consumption in Gallons / 100 mi driven ( or per 1000 mi driven for longer ownership periods ). At 36000 mi driven annually in a 48 mpg vehicle I use 750 gallons every year.

    The other benefit to me of the USAGE methodology is that it's easier to compute the cost of a trip or an annual usage. 36000 mi x ~2.1 GPC is ~ 750 gal x $4 a gallon = $3000 annual cost. Personal preference here.

    The KEY diffenence between the two systems is that the GPC method is a much more accurate metric for gauging consumption than the MPG method. For example...
    A Civic ICE driver getting 33 mpg on average switches to a Prius getting 48 mpg on average. BIG numbers!
    However that driver only reduced his usage by one third. Before he was using 30 gal to go 1000 miles, now he's using 21 gal to go the same distance. That is he saves 9 gal for every 1000 miles driven.

    Now taking the case of an ICE Expedition owner that switches to a Tahoe 2-Mode going from about 17 mpg on average to about 20.5 mpg on average in the T2M. Little numbers. Nowhere near comparable to the savings of the Civic to Prius switcher.
    However usage-wise the Expedition owner was using 58.8 gal to go 1000 miles previously whereas in the T2M he's now using 48.7 gal to go that same distance. He saves 10 gal for every 1000 miles driven.

    It's not so intuitive to see that the two switches are equivilent if one uses the MPG method.
    However using GPC it's easier to see and compute IMO.
    Civic ICE.. 3.0 GPC
    Prius ....... 2.1 GPC

    Exped ICE.. 5.8 GPC
    T2M........... 4.9 GPC
     
  8. qbee42

    qbee42 My other car is a boat

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    MPG is no better or worse than GPM. It is exactly the same data. One or the other may be more intuitive, depending on the circumstances, but that doesn't make the other false. Obviously in any calculation you must be careful of round-off errors and divide by zero situations.

    Tom
     
  9. patsparks

    patsparks An Aussie perspective

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    And switching from an Expedition to a Prius would reduce consumption by 37 gallons per 1000 miles which is almost as good as taking a Civic and half a Prius off the road over the same distance.
     
  10. KayakerNC

    KayakerNC Member

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    Since my Pri shows MPG, not GPM, I'll stick with the (old-fashioned, inefficient, and non-metric?) display.
    Whatever happened to all those KM speed limit signs?
    :car:
     
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