Study: Gallons Per Mile May Depict More Accurate Fuel Economy
June 23, 2008
DURHAM, N.C. — The auto market may evaluate fuel efficiency in terms of miles per gallon, but a recent study from Duke University's Fuqua School of Business has suggested that it may be more useful to consider the inverse: gallons per mile.
According to the report, which appears in the June 20 edition of Science magazine, Duke management professors Richard Larrick and Jack Soll demonstrated through a series of tests that consumers often have a misguided notion that gas consumption is reduced evenly as fuel efficiency increases.
During the study, consumers were given a series of choices where fuel efficiency was noted in mpg. However, they were not able to easily pick out which choice would have the greatest improvement in fuel efficiency.
For instance, when given the choice between going from 34 mpg to 50 mpg over 10,000 miles versus improving from 18 mpg to 28 mpg, most chose the former.
However, jumping from 18 mpg to 28 mpg actually saves twice as much gas (198 gallons) compared to an improvement from 34 mpg to 50 mpg (94 gallons), the study indicated.
But when respondents were given fuel-efficiency choices expressed in gallons per 100 miles, they made the correct pick.
For instance, 18 mpg changes to 5.5 gallons per 100 miles and 28 mpg becomes 3.6 gallons per 100 miles. With today's prices, that saves consumers roughly $8.
"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 explained.
Soll gave the example of swapping a large vehicle with a fuel efficiency of 10 mpg with one that earns 20 mpg, a move that reduces gas usage to five gallons per 100 miles instead of 10 gallons, saving the driver five gallons.
However, exchanging a 25 mpg vehicle for one that gets 50 mpg only reduces fuel consumption from four gallons per 100 miles to two gallons. This, obviously, saves just two gallons on the same 100-mile trip.
In other words, just because mpg is going up doesn't mean you'll save more gas.
"Miles per gallon is misleading and can play tricks on our intuitions," Soll commented. "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," he continued. "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."
Given the results of their study, Soll and Larrick urged consumer publications and automakers to report their vehicles' fuel economies in terms of gallons per 10,000 miles driven, which is common in other countries. According to the officials, this can better illustrate how much gas is used in addition to how much can be saved, they explained.
The following is a list of fuel efficiencies with the gallons consumed per 100 miles and gallons consumed per 10,000 miles:
MPG Per 100 miles Per 10,000 miles
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 |
