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Faculty & Research Home > Larrick Faculty Profile > Richard Larrick Home Page > Research Overview Page
Research Overview Page

The MPG Illusion

Original article and online supplement

Larrick, R. P., & Soll, J. B. (2008). The MPG illusion. Science, 320, 1593-1594. (subscription free)

Here is the standard subscription-only link to the article. Supporting Online Materials published by Science can be found here (with additional examples, descriptions of GPM, and more on research methods and results). This requires no subscription.

Brief summaries of the MPG illusion argument

http://mpgillusion.com/, Duke press release, Quiz, Video, and Science podcast

GPM calculator

We provide a calculator that converts MPG to GPM at this website. The calculator also allows you to compare different levels of MPG to see gas and cost savings.

Additional tools for converting MPG to gallons per mile (GPM) can be found here, including printable tables and excel-based calculators.

 

The case for GPM in a nutshell

One way to summarize our recommendation for GPM is with the following question: "Which is more useful to know: How far you can drive on a gallon of gas? Or, how much gas you will use while owning a car?"

MPG answers the first question. GPM answers the second question. We suspect that, when buying a car, most people want to know gas consumption. Gas consumption, as measured by GPM, can be directly translated to the cost of driving the car and to the amount of greenhouse gas emissions. MPG cannot.

Additional material on the MPG Illusion

The reason that MPG creates an illusion is because it is a ratio; by necessity, it has a curvilinear relationship with its inverse (GPM). Because people do not spontaneously take the reciprocal, they incorrectly map changes in MPG to changes in amount of gas consumed. The formula for calculating GPM in this graph is 10,000 miles divided by MPG. Download a powerpoint copy of this graph here. .

There are two main ways to reduce gas use (and greenhouse gas emissions) when driving: Improve efficiency and reduce miles driven. This graph is useful in quickly sizing up how the two factors can be traded off to reduce gas consumption. Click on the image for a pdf of two graphs plotting GPM as a function of distance and MPG. You can also download a powerpoint copy of the two slides here. (Note that the graph shown above is a cross section of the graph below set at 10,000 miles.)

The Green Grok (written by Dean Bill Chameides at Duke's Nicholas School of the Environment) has an excellent post on gas savings for different size mpg improvements over 1,000 miles. The accompanying figure shows the curvilinear relationship between MPG and gas savings. The figure looks at 3 starting levels of MPG and uses the current gas price of $4 per gallon (which are useful additions to the second figure in the Science paper).

Percentage increase in MPG can be a misleading measure for comparing fuel efficiency across vehicles. This note describes why it is prone to illusions and gives three illustrations of why it fails. Section II of the Supporting online materials at Science gives a detailed analysis of why "proportional" (or "percentage") reasoning errs.

This note on GPM and the metric system describes why adopting GPM would not be as hard as switching to the metric system, and why switching to the metric system would not solve the MPG illusion.

We think that gallons per 10,000 miles (GPM) is a useful nudge. The Nudge blog is updated frequently with interesting suggestions for how to improve decision making.

Converting MPG to GPM (these links also appear on http://mpgillusion.com/)

A web-based GPM calculator to calculate gas use and gas savings.

Tables of GPM measures - Print this sheet to see how GPM measures correct the MPG illusion. It is also useful for calculating gas consumption when buying a new car. Note: MPG numbers in Table 2 are rounded to the nearest .5. The formula used to create these tables is distance (either 100 miles or 10,000 miles) divided by MPG.

If you are interested in making GPM calculations for yourself, these excel files will do the math for you. They open in a new window and can be downloaded and saved to your computer. The formula in both worksheets is simply distance divided by MPG:

GPM calculator for one car (a downloadable excel file)

GPM calculator for two cars (a downloadable excel file)

The tables at these two links show the gas (and dollar) savings from the current Cash for Clunkers program proposed by Rep. Betty Sutton of Ohio (see this link at www.mpgillusion.com for more details):

Cash for Clunkers Table of Gas Savings

Cash for Clunkers Table of Dollar Savings

The graphs in the link below (which also appear above) are an easy way to compare how different combinations of distance and MPG affect gas consumption. Click on the image to open a pdf copy (or click here for a powerpoint copy). Note: The file contains two slides.

See more complete calculators for combinations of distance and MPG at these two sites (we did not construct these calculators and have not used them extensively):

http://www.revolutioninmotion.com/fuel-cost-calculator.htm

http://raja.gbc.googlepages.com/home (at raja gopalan's July 14 post on http://goodrepublicanusa.blogspot.com/)

See Edmund's Green Car Advisor on calculating gas-guzzler trade-ins and CO2 emissions.

Teaching Materials

Teaching notes, powerpoint slides, and additional topics

Selected Research Summaries

Duke press release (published at Autochannel.com, MotorsToday.com, ScienceDaily, Automotive.com, escience news, )

Video (Also posted at Duke)

Recorded interviews

Science podcast , NPR's All Things Considered, NPR's Science Friday (about 15 minutes in)

Selected Print and Online Stories

New York Times Science, National Geographic, US News, Consumer Reports, Associated Press (syndicated - MSNBC, USA Today, Seattle Times, Detroit Free Press, Washington Times, Forecast Earth, etc., partial list), Guardian (syndicated), Reuters (syndicated - Scientific American, ), Asian News International (syndicated), Bloomberg, Toronto Star, APA Monitor, Google comment, Manila Times, Monterey Herald, BusinessGreen, Voice of America, Air America, News & Observer

Selected Media/Academic/Automotive/Environmental Blogs

Edmunds, New York Times Auto, Livemint, WSJ The Numbers Guy, LA Times Auto, CNN SciTech Blog, Nudge, Predictably Irrational, Cars.com, CNGcar, Sustainable Lawrence, Real Climate, Sierra Club-The Green Life, Sierra Club - Compass,Sierra Club - Spaces, New Scientist, Tree Hugger, Gristmill, Discovery, Motor Daily, Mongabay, American Mathematical Society-Math Digest, Oxford Analytica, Decision Science News, Knight Science Journalism, The Consumerist, The Center for Cost-Effective Consumerism, Conde Nast Portfolio-Market Movers, Maine Today, Mother Jones, DestinationCRM, Autoblog, Boston Globe's Greenblog, AutoblogGreen, The Car Connection, Crosscut.com, HybridCars, Riverwired, Greenrightnow.com, Science and Law Blog, MIT Technology Review, Driive, Sightline, Makezine.com, Greenweb, Next Autos, Kicking Tires, Slog.thestranger, Michigan innovators, Environmental Protection, The Truth About Cars, World Business Council for Sustainable Development, Greenwire, Environmental News Network, Energy Efficiency News, Greendaily, Engines of our Ingenuity, Low CVP, Mental Floss,

Selected Personal Blogs

The Digerati Life, Science Geek Girl, MetaSD, Bunnie's Blog, The Science Pundit, Steve Krause, Finance Clippings, Mileages, BBB Consumer Education Blog, Fiddling Marcus, Everyday Scientist, The Oyster's Garter, Quietly Making Noise, Forgotten Aria, Graceful Flavor, Welcome2Green, A Musing Environment, Matt-Helps, Dr. Bortrum, Cubalicious, Fuel Efficient Autos, Wallet Pop, New Deadalus, Aces Full of Links, MPG Blog, blogan.net, The BizOp News, 12 Angry Men, Reasonable Rant, Stats Made Easy, Pure Pedantry, Econstudent, Home Efficiency Blog, Arfully Underemployed, Pocketmint, Green Daily, Mass Eyes and Ears, Dear Science, Lunchtime Longhouse, Verda Vivo, Neural Transmissions, Qurbit, Keifus Writes, Asymptotic Life, Thinking on the Margin, Hope is Not a Plan, New Energy and Fuel, Greenopolis, CareyTilden, My Blog, Your Money, Robert A. Green, An Inconvenient Blog, Restart2, Ecoservices, Douglas Purdy, Fuelishness, Finallygreen, Snafu, Knowledge and Experience, Natural Path, Info Is Free, Lady Blog, Life By Design, Streams of Consciousness, Iowa.BarackObama, It's the Thought that Counts, bMighty, Ecomerge, My Crazy Life, Rain Forests in Crisis, Geek Noise, The Blog at the End of the World, The Thief,

 

"When the time is ripe for certain things, these things appear in different places" (Farkas Bolyai)

Eric De Place at Sightline Daily posted here and here, noted by Andrew Sullivan.

Barry Nalebuff and Ian Ayres in Forbes (Why Not?), Dennis Simanaitis at Road and Track here (see halfway down) and here, Automotive News, Boston Herald, Ben Garrido at Reno News & Review, Tony's Climate Blog, Halfbakery, No, Dave, it's just you,

A CarTalk puzzler that illustrates the problem with MPG

Extensions to other Environmental Decisions

The problem with SEER ratings for air conditioners here and here

Transportation in the Context of All Greenhouse Gas Sources

Eric Hess has a short post at Sightline Daily on a figure from the World Resources Institute that breaks down greenhouse gases by source. The graph shows that agriculture plays as large a role as transportation in greenehouse gas emissions. Although it does not break it down this way, it should be noted that meat production is a major source of the agricultural contribution of greenhouse gases: Feed is grown (using fertilizer) and then "converted" to meat, which is an inefficient process of using resources to produce food; the livestock themselves produce methane.