The MPG Illusion: How Cognitive Biases Increase Climate Change

Richard Larrick (Duke University)


Please note: Instructors are welcome to use or adapt these teaching ideas for their own classes, provided the use is noncommercial and appropriate credit is given.


To: (1) facilitate a discussion of the link between psychology and climate change, and (2) offer a specific example of how reframing a problem can help consumers make more environmentally sustainable choices


Which improvement in fuel efficiency saves more gas over the same distance driven: replacing a vehicle that gets 10 MPG (miles per gallon) with one that gets 11 MPG, or replacing a vehicle that gets 35 MPG with one that gets 50 MPG? Most people fall prey to the "MPG illusion" and guess that upgrading a 35 MPG vehicle to 50 MPG will save more gas, but in reality the upgrade from 10 MPG to 11 MPG has more impact. In an action teaching activity that explores the link between psychology and climate change, students are first asked to rank the impact of different MPG upgrades in fuel efficiency, and they then learn (1) how the MPG illusion works, and (2) what can be done to avoid the illusion and make more environmentally sustainable choices.


This teaching module offers an example of how psychology can be used to address climate change. The activity focuses on a cognitive bias my colleagues and I refer to as "the MPG illusion," and the module is easy to incorporate into courses on decision making, consumer behavior, environmental psychology, applied psychology, statistics, research methods, and related fields.

When automobile fuel efficiency is expressed as "miles per gallon" (the common measure used in the United States), most people underestimate the savings from replacing inefficient cars. People incorrectly believe that gas savings and greenhouse gas reductions are a linear function of increases in MPG. However, the actual relationship is curvilinear. The table below shows the curvilinear relationship by translating different MPG levels into gallons of gas consumed when driving 10,000 miles.

GPM = Gallons Per 10,000 Miles
10.0 MPG  =  1000 GPM
11.0 MPG  =  900 GPM
12.5 MPG  =  800 GPM
14.0 MPG  =  700 GPM
16.5 MPG  =  600 GPM
20.0 MPG  =  500 GPM
25.0 MPG  =  400 GPM
33.0 MPG  =  300 GPM
50.0 MPG  =  200 GPM

The math makes clear that the gains from 10 to 11 MPG, 16 to 20 MPG, and 33 to 50 MPG all save roughly 100 gallons per 10,000 miles, which is equivalent to one ton of carbon. As people weigh the decision to replace a current car with a more efficient car, they are likely to underestimate the large cost and greenhouse gas savings of seemingly small improvements on inefficient cars. The MPG illusion is corrected when fuel efficiency is expressed in terms of GPM such as "gallons per 10,000 miles."

One good way to introduce the MPG illusion is by having students take a quiz in which they compare the gas savings and greenhouse gas reductions of replacing one car with a more efficient car. Additional materials include a video, website, PowerPoint slides, and short readings that can be used to explain the illusion and the math in detail.

The module is especially well-suited to introduce a lecture or discussion on the link between psychology and climate change. In the case of the MPG illusion, a clear policy implication is to have the government and consumer magazines supplement MPG information with GPM measures (e.g., carbon emissions per 100 miles). More generally, decision makers need clear information about the greenhouse gas consequences of their transportation, diet, work, and housing choices — that is, the main contributors to their "carbon footprint." Students can be challenged to think of better tools for helping people make environmentally sustainable decisions.

The website contains a link to a GPM calculator that offers information on different cars (updated each year as new car models come out). This calculator provides a practical tool for students and others to compare fuel efficiency across cars, and was designed as the sort of tool that the U.S. Environmental Protection Agency ( and consumer magazines could offer to correct the MPG illusion.

In sum, learning about the MPG illusion can help students make better decisions for themselves and see the link between psychology and climate change.

Demonstrating the MPG Illusion

A classroom discussion of the MPG illusion should begin by having students take a brief quiz to test their intuition about MPG. There are two main options for administering a quiz:

Online Quiz

With this option, students complete a one-question online quiz prior to class. The online quiz asks students which alternative will save more gas: replacing a 10 MPG vehicle with a 20 MPG vehicle, or replacing a 20 MPG vehicle with a 50 MPG vehicle. Most students choose replacing the 20 MPG vehicle with a 50 MPG one. However, this choice saves only 3 gallons per 100 miles, whereas the other choice saves 5 gallons per 100 miles.

To help students understand the MPG illusion, they can be instructed to follow up the quiz by viewing the video below, by visiting the MPG illusion website, or by reading the original Science report and its supporting materials. Instructors can then build on this material in a lecture or classroom discussion. 

In-Class Quiz

The in-class quiz, based on Study 1 of Larrick and Soll (2008), can be distributed as a handout, given on a lecture slide, or collected prior to a lecture if the instructor wants to present a summary of student responses.

Rank what's best for the environment:
(A) Upgrading mileage from 18 to 28 MPG
(B) Upgrading mileage from 16 to 20 MPG
(C) Upgrading mileage from 34 to 50 MPG
(D) Upgrading mileage from 20 to 22 MPG
(E) Upgrading mileage from 42 to 46 MPG

The quiz asks students to assume that a person drives 10,000 miles per year and is considering an upgrade to a more fuel efficient vehicle. They are asked to rank five vehicle upgrades in order of their benefit to the environment, using "1" for the largest gas savings and "5" for the smallest. Students most often rank the upgrades according to the size of linear improvement (C first, followed by A, E, B, and D). In reality, however, the correct order is A-E. For instance, A and B both save more gas than C.

After the quiz, instructors may wish to use or adapt relevant PowerPoint slides to explain how MPG can be misleading and to start a discussion about better measures for making greenhouse gas decisions. In addition, students might be assigned a follow up reading such as those mentioned earlier. 

Tips on Implementation

Here are a couple tips based on my experience teaching about the MPG illusion:

  • It is important that students complete the quiz before reading or listening to an explanation of the illusion. If instead students learn about the illusion before experiencing it themselves, the teaching module may not have as much impact as it would otherwise.

  • When students are faced with a choice such as replacing an 18 MPG car with a 28 MPG car versus replacing a 34 MPG car with a 50 MPG car, they sometimes ask, "Why not replace the 18 MPG car with a 50 MPG car?" In response, instructors might point out that even though this choice would yield the greatest savings in gas, the MPG illusion shows that consumers who can't afford highly efficient cars can still have a large effect by modestly upgrading fuel inefficient vehicles.

Variations and Extensions

This teaching module can be broadened in several ways. For example, instructors might describe other psychological biases that affect climate change, or might ask students to write a paper linking psychological biases to human impact on the environment. Alternatively, instructors might discuss how problem framing affects consumer choices beyond car buying. For additional suggestions and materials, see:


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