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

Laura Brinn of Duke University, +1-919-660-2903, laura.brinn@duke.edu