• Jun 20th 2008 at 7:58PM
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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]

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    • 1 Second Ago
      • 7 Years Ago
      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.
      • 7 Years Ago
      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.
      • 7 Years Ago
      "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.
      • 7 Years Ago
      I have always been curious about Americans' attachment to the imperial system. When all the former English colonies started going metric in the 60s and 70s, America started too, but I guess the public decided they didn't want that. So, aside from people being in an international or scientific environment, and businesses being in a slow move to harmonize goods being sold abroad, my impression is that the average American gets very little exposure to metric. I've wondered why. Although many former English colonies had initial resistance (old habits are hard to kill, after all) and there has always been resistance in England (they, after all, invented this confusing system), but I've always wondered about America. They declared independence from England long, long before, say, Canada (where I'm from) and was even willing to take up arms against England if the Brits put up any resistance. In Canada, incidentally, even though we went officially metric in the late 70s, imperial lingers, largely due to our close proximity to America. For instance, school curriculum is metric and road signs are metric, however, the construction industry largely is not, especially for the small mom-and-pop businesses. I understand other countries like Australia and South Africa are more metric than Canada.
      • 7 Years Ago
      "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."

      Get over it. Divide "litres (or liters) per 100 km" into 282 to get the equivalent MPG imperial.

      Divide into 235 to get the equivalent in U.S. gallons. It's not rocket science and can be done on any pocket calculator (or cell phone).
      • 7 Years Ago
      Metric minds are more efficient.
      • 7 Years Ago
      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.
      • 7 Years Ago
      After reading Second Opinion's post, I need to clarify why I agree with flipping the order in which the type of units fall (from distance per volume to volume per 100*distance) and why I suggest that we take the opportunity to start using metric (liters and kilometers rather than gallons and miles).

      For me, it's not so much "Euro-worship" as it is respect for simpler means of measure and comparison.

      To clarify, I would rather that US vehicle buyers be able to EASILY compare our efficiency to vehicle efficiency in other markets. This means using both the order and the units used by most car markets: L/100 km.

      To refer/respond to Second Opinion's response on his(?) own blog: while we in the US tend to be used to "bigger is better" we are also used to "smaller is better" such as when we see gas sold at two stations on opposite street corners at $3.95/gal and $3.99/gal.

      From reading a few posts on the blog blog, Second Opinion seems to be a smarter and more thoughtful shopper than the average person (as classical liberals tend to be). However, Second Opinion does not seem to take into account that most shoppers buy on emotion rather than hard numbers or the meaning of numbers. It is easy to tell someone that going from 75 mpg to 100 mpg is as significant a gain in efficiency as the 25 mpg to 50 mpg jump. To most consumers, 25 mpg is 25 mpg. When you use the volume/100*distance model, the buyer sees that that ton of extra money that goes into the 75-100 mpg jump has a lot less effect than the 25-50 jump or the 10-35 mpg jump. Second Opinion, remember that you are the exception to the rule: most people are easily snowed by logarithmic scales and inverse scales; most people do not have the math skills that you have. Second Opinion, remember that it's easy to forget that not everyone is as smart as you.

      And so, for easier comparison between our market and others and for easier comparison on a flat scale, I say we should go to L/100 km for fuel efficiency.
      • 7 Years Ago
      .... 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...
      • 7 Years Ago
      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.
      • 6 Years Ago
      I like this Blog very much and I would like to share more info about this topic: Washington DC Limo - BWI Limo - Baltimore Limo
      • 7 Years Ago
      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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