Forget MPG - let's move to GPM
Photo by Laffy4k. Licensed under Creative Commons license 2.0.
There's already a lot of calculation needed to figure out how efficient a vehicle is. Here in the U.S., the government tries to help by publishing official MPG numbers of the various models offered for sale. But these numbers just muddy things up, say two management professors - Richard Larrick and Jack Soll - over at Duke University. Thinking in terms of miles per gallon doesn't give people a good understanding of a vehicle's real efficiency when compared to other vehicles.
In Europe, for example, mileage figures are given in liters per kilometers. There are online calculators we can use to figure out what the MPG equivalents are, but the Duke profs think we should be moving to a gallons per mile model here in the U.S. That way, people might begin to see that trading a 14mpg SUV for a 21mpg hybrid version, for example, saves more fuel than trading in a 35mpg sedan for a 50mpg Prius. Right now, very few consumers realize this when they're out debating which car to buy. Sure, a Prius burns less fuel than a SUV hybrid over the same distance, no question, but there's something to be said for the seemingly mediocre fuel economy improvements made in the low-mpg segments. From the pres release pasted after the jump:
- Most people ranked an improvement from 34 to 50 mpg as saving more gas over 10,000 miles than an improvement from 18 to 28 mpg, even though the latter saves twice as much gas. (Going from 34 to 50 mpg saves 94 gallons; but from 18 to 28 mpg saves 198 gallons).
This is something we've pointed out on AutoblogGreen before, but we're glad to have some research on public (mis-)understanding of the numbers to refer back to in the future. The main point is that we need to prioritize getting the most inefficient vehicles off the road, not trying to get everyone into a hyper-efficient hybrid. You can listen to a three-minute interview with Richard Larrick on NPR.
Press Release:
Gallons Per Mile Would Help Car Shoppers Make Better Decisions
DURHAM, N.C., June 19 /PRNewswire-USNewswire/ -- Posting a vehicle's fuel efficiency in "gallons per mile" rather than "miles per gallon" would help consumers make better decisions about car purchases and environmental impact, researchers from Duke University's Fuqua School of Business report in the June 20 issue of Science magazine.
Inspired by debates they had while carpooling in a hybrid car, management professors Richard Larrick and Jack Soll ran a series of experiments showing that the current standard, miles per gallon or mpg, leads consumers to believe that fuel consumption is reduced at an even rate as efficiency improves. People presented with a series of car choices in which fuel efficiency was defined in miles per gallon were not able to easily identify the choice that would result in the greatest gains in fuel efficiency.
For example, most people ranked an improvement from 34 to 50 mpg as saving more gas over 10,000 miles than an improvement from 18 to 28 mpg, even though the latter saves twice as much gas. (Going from 34 to 50 mpg saves 94 gallons; but from 18 to 28 mpg saves 198 gallons).
These mistaken impressions were corrected, however, when participants were presented with fuel efficiency expressed in gallons used per 100 miles rather than mpg. Viewed this way, 18 mpg becomes 5.5 gallons per 100 miles, and 28 mpg is 3.6 gallons per 100 miles -- an $8 difference today.
"The reality that few people appreciate is that improving fuel efficiency from 10 to 20 mpg is actually a more significant savings than improving from 25 to 50 mpg for the same distance of driving," Larrick said.
Soll noted that replacing a large vehicle that gets 10 mpg with one that gets 20 mpg reduces gas use per 100 miles from 10 gallons to five, a 5-gallon savings. Replacing a small vehicle that gets 25 mpg with one that gets 50 mpg reduces gas use per 100 miles from 4 gallons to 2, a saving of only 2 gallons.
"Miles per gallon is misleading and can play tricks on our intuitions," Soll said.
"For families and other owners of more than one type of vehicle, the greatest fuel savings often comes from improving the efficiency of the less efficient car," Soll added. "When fuel efficiency is expressed as gallons per 100 miles, it becomes clear which combination of cars will save a family the most gas.
"We believe that everyone should try to be as fuel efficient as possible. For some people, that may mean driving the most efficient car available, such as a small hybrid car, but for others it may mean finding the most efficient option possible within their chosen class of car," Soll said. "There are significant savings to be had by improving efficiency by even two or three miles per gallon on inefficient cars, but because we communicate in miles per gallon, that savings is not immediately evident to consumers."
The authors recommend that consumer publications and car manufacturers list efficiency in terms of gallons per 10,000 miles driven, which is already the standard in many other countries. "This measure makes it easy to see how much gas one might use in a given year of driving and how much gas, and money, can be saved by opting for a car with greater efficiency," Larrick said.
Larrick and Soll's research was funded by Duke University.
Miles Per Gallon Gallons Consumed per Gallons Consumed per
100 Miles Driven 10,000 Miles Driven
10 10.00 1,000
15 6.67 667
20 5.00 500
25 4.00 400
30 3.33 333
35 2.86 286
40 2.50 250
45 2.22 222
50 2.00 200
[Source: Duke University, NPR]
Reader Comments (Page 1 of 2)
Dad 7:48AM (6/21/2008)
"Right now, very few consumers realize this when they're out debating which car to buy."
These educators should worry about the terrible job educators are doing with math and simple critical thinking.
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why not the LS2LS7? 8:25PM (6/20/2008)
No system of measurement is going to suddenly make people good at math.
If this system makes it easier to know that an improvement on an SUV saves more gas than a Civic, but it will make it more difficult to figure out how many gallons of gas you need to go a certain distance (unless it's a multiple of 100 miles).
Right now, with may car getting 21mpg, I know if I want to go 40 miles, I need two gallons. Or if you want to know how far you can go on a full tank.
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rj 8:27PM (6/20/2008)
I see even Canadian dealerships advertise the mpg of trucks - which is an extra confusion here because they use Imperial gallons (I think) On top of that we are subject to a lot of US advertising which of course uses US gallons.
liters per 100km is much better - but you have to get used to it. My Corolla gets about 7.
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Doug 9:18PM (6/20/2008)
I'm for anything that uses the metric system. Really wish we could have managed that transition a long time ago.
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dhofmann 9:18PM (6/20/2008)
Better yet, MJ (or BTU or kWh) per mile, so people can compare between different fuel types. (And change gas pumps to charge by the same unit of energy.)
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rgseidl 11:06PM (6/20/2008)
It's actually even more complicated. First, you cannot compare different fuels by volume. A gallon of diesel contains ~10% more energy than a gallon of gasoline, whereas a gallon of E85 contains ~25% less. Direct comparisons with electric cars are not trivial: do you consider just the kWhs consumed at the wall outlet or rather, the primary energy required to produce them?
Second, the mileage estimates depend on a driving cycle. EPA changed that this year for the purpose of consumer information shown in dealerships, but CAFE and gas guzzler tax are still calculated on the basis of the old, more lenient cycle. Moreover, there is a giant loophole in CAFE that rewards sales of flex-fuel vehicles even where E85 is not available.
Third, plug-in hybrids completely break the current system of MPG certification. Marketing departments make bold claims of triple-figure MPG performance but they are essentially meaningless because they are based on proprietary drive cycles. The battery state of charge at the beginning of the test has a huge impact on the consumption rate of the liquid fuel, especially if the drive cycle is shorter than the all-electric range. Grid electricity consumption in MPkWh (or reciprocal) is usually not given at all because electricity is dirt-cheap in the US. That is not the case in much of Europe. Besides, it has to be produced somehow, so there are indirect effects such as remote CO2 emissions and/or radioactive waste production that depend on the grid mix, which varies by country and even by region. Marketing departments conveniently ignore these as well.
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Doug 12:08AM (6/21/2008)
Cost per mile is probably a better all-around metric. If we're talking about using electric/gas hybrids, alternative fuels (hydrogen, CNG, ethanol, butanol, green diesel, whatever) it's probably a metric that evens out the playing field. Each of these options have different ramifications on their individual productivity measures, but their cost / mile driving reflects how much the customer will have to pay over the life of the vehicle: say 150,000 miles at ~ $0.10/mile means $15,000 of energy. My Honda Accord is around $0.15 cents per mile. I think things need to be in the $0.05/mile if we're going to be minimizing pollution and/or most efficiently using something relatively expensive (hydrogen, fossil fuels).
Just a thought.
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Mark 12:08AM (6/21/2008)
Of course, if we made sure that everyone passed high school algebra we might not have a problem. And just to be sure, we should add some fuel consumption problems to the rate problems. How about a choice between replacing half of a fleet at X percent improvment or the whole fleet at Y percent (where Y > X)?
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Karl-Uwe Strunzen 12:40AM (6/21/2008)
.... or how about something really obscene like
litres/100 km
isn't the idea of an international standard (and this doesn't apply only to units of measurement) that we all go by the measuring stick. wouldn't it be time now to let go of the [pounds/(inches-squared)]*cubic-sneezes .....?
i've even seen things like grams/mile used ....
to ISO or not to ISO,
aye, that be the question...
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Karl-Uwe Strunzen 12:41AM (6/21/2008)
.... or how about something really obscene like
litres/100 km
isn't the idea of an international standard (and this doesn't apply only to units of measurement) that we all go by the measuring stick. wouldn't it be time now to let go of the [pounds/(inches-squared)]*cubic-sneezes .....?
i've even seen things like grams/mile used ....
to ISO or not to ISO,
aye, that be the question...
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terrence_bethea 10:11PM (6/21/2008)
It doesn't matter how you do the math. The fundamental logic is flawed.
It's just plain dumb to trade an 18mpg Explorer for a 28mpg Escape Hybrid if all you really need for transportation is a 32mpg Camry or a 50mg Prius.
In essence, it matters a lot where you start but it also matters how far you are willing to go. Sure, we want EVERY Excursion owner to drive something else. In that case it almost doesn't matter b/c that vehicle sux. But it's nonsensical to compare that to the Camry owner that goes to a Camry Hybrid. It's just plain common sense that the more you waste from the beginning, the greater the benefit from changing vehicles.
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jcwinnie 8:41AM (6/21/2008)
While cost per mile is a better metric, it unfortunately omits a key consideration, i.e., cost to life on the planet as we know it. Simply measuring cost to your wallet or pocketbook encourages drilling in the Alaskan National Wildlife Reserve, off-shore, in the melting Arctic, etc., to keep those SUVs (albeit now slightly more efficient) running to the mall.
We should properly look at cost in terms of pocketbook, yes, and cost in terms of energy used (reward efficiency) and cost in terms of carbon footprint, all should be quickly accessible.
Just looking at gallons per mile also stays with in the ICE paradigm, i.e., we continue to think only in terms of gallons of gas or diesel. On the other hand, the Duke professors are correct in their observation that GPM provides a missing perspective. Funny that, eh?
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Nils 9:05AM (6/21/2008)
Metric minds are more efficient.
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GoodCheer 9:43AM (6/21/2008)
I add my vote to Karl-Uwe Strunzen's:
There is a widely used metric that addresses the concerns of these Duke guys. Our national pride (in being the last holdout to use the BRITISH units of measurement) should not get in the way of adopting a better system.
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T2 10:48AM (6/21/2008)
Replacement with the metric litre is useful since there is a confusing disparity between the British and American gallon and always will be.
For political reasons announcing price increases on the litre - being a quarter of a gallon - are correspondingly smaller and therefore more palatable. For the same reason, I've noticed that marketing at the greengrocery depts of supermarts prefer to price items by the Lb rather than pricing by the larger kilogram. Packaged produce from the USA has to be sold metrically by law. Our american friends can't sell in pounds here, so bowing to us natives, they sell in very convenient quantities of 454gms instead !
A humorous statement today but it may bite USA exporters in future since the locovore movement knows that 454gm of bacon was imported but the 500gm package is more likely to be local.
Working in metric with EVs regarding electric motors/gear ratios/drawbar pull/acceleration/ regen efficacy etc These are much easier to work out in the "nongravitational units of the metric system. The imperial system uses the pound for mass and force whereas the metric makes a huge distinction between mass in kgs and force in Newtons. I agree with Nils, metric certainly promotes clarity of thought.
T2
Secondly price inreases on trhe litre are easy to bear being s
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Bill 1:17PM (6/21/2008)
T2 wrote (in comment 14):
"Replacement with the metric litre is useful since there is a confusing disparity between the British and American gallon and always will be."
Yeah, but we'll still be disagreeing on how to spell it! :-)
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David Cabral 3:24PM (6/21/2008)
It make sense to buy a used sedan like a Toyota Corolla instead of a brand new Prius.
Most people don't care about liters per 100 km, gallons per 100 miles, or miles per gallon. I instead suggest dollars per year.
Use this equation:
Dollars spent on gasoline per year is equal to
A multiplied by B, then divide by C
A = miles per year that you drive (for some people, this is 20,000; for others, it could 10,000 or 5,000).
B = Dollars per gallon price of gasoline (or diesel if you are looking at a diesel vehicle). for comparison purposes, I suggest using a value of 5 for B (since it is possible for the next 5 years we could see $5 per gallon gas).
C= miles/gallon of vehicle.
You will then see how much $$ you would spend on gas per year. Is a hybrid worth it? depends on your case... good luck
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Chris 3:34PM (6/21/2008)
The "research" done by the Duke professors is flawed and skewed. While it is true that swapping out a 14 mpg SUV for a 21 mpg SUV over a given number of miles is more beneficial than swapping out a 35 mpg sedan for a 50 mpg Prius, most people are not faced with that option in real life. Most people are faced with replacing a given vehicle, not a choice of two different ones, and even if they had two different ones to chose from to replace, the chances of them being driven the same is absurdly small. The SUV may be driven 4000 miles per year and the sedan 16000 miles per year. Then upgrading the sedan to a Prius would be a far better choice for that individual, even though upgrading the SUV may have appeared better over a fixed interval.
Further, mpg vs. gpm is the same thing, just inverted. If you look at a Picasso upside down, its still a Picasso! Changing to gpm would not change peoples perceptions of fuel efficiency improvements because the ration from vehicle to vehicle will still come out the same!
Though standardizing the measurement systems, ratios and comparisons would certainly go a long way toward getting rid of some of the inherent confusion, what really needs to happen is people need to care about their actions enough to take a moment to consider what they are doing, their vehicle purchase in this case, and take a couple of minutes to figure out what would really be the most beneficial in their particular case. There is no cure-all, one-size-fits-all answer that everyone can use because we are all different and have different circumstances. We can give them the best tools on earth, but they have to want to use them or else the tools are useless.
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anym_avey 7:39PM (6/21/2008)
Liters per 100km has to be one of the most useless metrics ever invented, and where it fails is that neither of the units involved is ever "1.0", except perhaps by freak coincidence when evaluating motorbikes or SmartCars.
For example, my present vehicle could be described as achieving about 7L/100km. What does that mean if I'm planning to take an 18km trip? Absolutely nothing without scratching out some algebraic equations for a few minutes, unless perhaps I decide that 18 is close enough to 20, which is a fifth of 100, and 1.4 is a fifth of seven, so I'll probably consume around 1.5L for the trip.
On the other hand, if I know my car gets about 32mpg, and I have a 12 mile trip coming up, I can figure that 12 is a little under a third of 36 and assume I'll use, including starting and stopping penalties, about a third of a gallon of gas.
Why was this calculation faster? Because it took the smaller of the two units -- which will always be the volume of gas consumed for any unit so large as a liter or a gallon -- and set it to "1".
I also think the respondants to that Duke survey were smarter than the eggheads gave them credit for. They weren't thinking in terms of percentage increases; they were looking at how far one can travel on a given amount of gas. At the end of the day, that's pretty much the goal of all transportation, so obviously getting 50mpg is the best-case scenario of the given options. Nor is this mode of thinking likely to skew people's perceptions when buying a vehicle -- if they need some sort of wagon-type vehicle and gas is expensive, the Tahoe gets traded in for a hybrid Highlander regardless, beacuse it will be cheaper to operate.
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comatus 8:57PM (6/21/2008)
Viewed upside down, a Picasso is still a result of the metric system. By all means, let us abandon a system based on the human frame and pace, and adopt the oh-so-scientific and rational schema based on a mismeasurement of the size of one planet. Brought to you by the same efficient rationalists who invented the draft, margarine, the ten-month calendar, and the guillotine. If the Europeans are so much better at everything, why is everything so much worse in Europe?
Innumerates are still innumerate in metric. And the Prius is that ugly on purpose. Its devotees would not care if it got 10 MPG (or 12 L/Kkm), so long as its styling told the world "I hate myself." Sooner or later senators, elephants or no elephants, Carthage must be destroyed.
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