You Don’t Understand Fuel Economy; Blame MPG

Jun 20th, 2008 | By | Category: Environmental

Assuming you drive the same miles per year, which change will save more gas in a given year:

* Switching from a Dodge Ram at 13 MPG to a Toyota Tundra at 15 MPG

* Switching from a Honda Fit at 32 MPG to a Toyota Prius at 44 MPG.

(Mileage figures are from Consumer Reports.)

Have your answer? Ok, next question.

Assuming you drive the same miles per year, which change will save more gas in a given year:

* Switching from a Dodge Ram that needs 770 gallons per 10,000 miles, to a Toyota Tundra that needs 667 gallons per 10,000 miles.

* Switching from a Honda Fit that needs 313 gallons per 10,000 miles, to a Toyota Prius that needs 238 gallons per 10,000 miles.

Did your answer change?

As a measure of fuel economy, miles-per-gallon is incredibly unintuitive. One must consider both the change and the starting point when deciding the significance of an increase in MPG. Nasty.

How nasty? Richard P. Larrick and Jack B. Soll collected data to discover just how confused people become when considering changes in miles-per-gallon. Their work was just published in the Journal Science.

The most telling passage from the study:

The study was presented in an online survey to 171 participants who were drawn from a national subject pool. Participants ranged in age from 18 to 75, with a median age of 35. All participants were given the following scenario (5): “A town maintains a fleet of vehicles for town employee use. It has two types of vehicles. Type A gets 15 miles per gallon. Type B gets 34 miles per gallon. The town has 100 Type A vehicles and 100 Type B vehicles. Each car in the fleet is driven 10,000 miles per year.” They were then asked to choose a plan for replacing the original vehicles with corresponding hybrid models if the “overriding goal is to reduce gas consumption of the fleet and thereby reduce harmful environmental consequences.”

One group of 78 participants was randomly assigned to a policy choice framed in terms of MPG. They were asked to choose between two options: (option 1) replace the 100 vehicles that get 15 MPG with vehicles that get 19 MPG and (option 2) replace the 100 vehicles that get 34 MPG with vehicles that get 44 MPG. Note that town fuel efficiency is improved more in option 1 (by 14,035 gallons) than in option 2 (by 6,684 gallons). As expected, the majority (75%) of participants in the MPG condition chose option 2, which offers a large gain in MPG but less fuel savings [95% confidence interval (CI) = 65 to 85%].

Participants in the GPM condition (n = 93) were given the same instructions as those in the MPG condition. In addition, they were told that the town “translates miles per gallon into how many gallons are used per 100 miles. Type A vehicles use 6.67 gallons per 100 miles. Type B vehicles use 2.94 gallons per 100 miles.” They read the same choice options as used in the MPG condition, including the MPG information, but with an additional stem that translated outcomes into GPM for the hybrid vehicles [(option 1) replace the 100 vehicles that get 6.67 gallons per 100 miles with vehicles that get 5.26 GPM and (option 2) replace the 100 vehicles that get 2.94 gallons per 100 miles with vehicles that get 2.27 GPM]. As expected, the majority of participants (64%) in the GPM frame chose option 1, which offers a small gain in MPG but more fuel savings (CI = 54 to 74%). Overall, the percentage choosing the more fuel-efficient option increased from 25% in the MPG frame to 64% in the GPM frame (P < 0.01).

When talking about fuel efficiency in terms of gallons per mile, people were nearly three-times as likely to make the rational choice as compared to the same numbers in miles-per-gallon. Remember this when making your next car purchase.

Or, for the graphically minded, like me:

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