The MPG Illusion is confusing to car buyers – even after someone explains it.

Take a look at the above fuel efficiency label? Can you understand it? Do you think other people can?

Last week, the Environmental Protection Agency released two options for replacing the current fuel efficiency stickers displayed on new car windows. Before releasing these options, the EPA conducted an online survey and sixteen focus groups held around the country, the results of which the Nudge blog has been looking through.

Hopefully, you first noticed the two numbers at the bottom of the label, 4.5 and 3.3. Unless you’re buying a Ferrari, those probably seem too low for a standard miles-per-gallon statistic, right? You look closer and notice that they aren’t MPG numbers at all; they are GPM (gallons per mile) numbers.

GPM is a statistic in the news, thanks to work on the MPG Illusion, which shows that people misunderstand the non-linear relationship between gallons of gas consumed and distance traveled. One of the major implications of this research is that it obscures the value of improvements as fuel efficiency improves. People tend to undervalue small mpg improvements on inefficient gas guzzlers, and overvalue large jumps between two fuel sippers, like a Honda Civic and a Toyota Prius.

There have been many proponents of a new GPM metric, and the New York State Senate recently passed a law requiring it in car dealership showrooms. As part of its research, the EPA investigated consumer response to the concept. For the moment, the EPA found that consumers struggle with the MPG illusion, even when it is explained to them. For those who were able to understand the concept, they still expressed a preference for MPG over GPM because they were used to thinking in MPG terms. The EPA concluded:

It may be said that understanding the MPG illusion is extremely difficult to achieve and does not necessarily lead people to switch to a different type of vehicle nor does it make them prefer gallons per 100 miles over MPG. In essence, people prefer familiarity over facts.

Focus group respondents found the label shown above particularly confusing, not just because of the GPM statistic, but because it is presented in the slider in the upper right-hand corner with a range of 2 (best) to 10 (worst). Without a general knowledge of gas guzzler and hybrid GPMs, the scale made little sense. In the end, the EPA decided to continue using MPG estimates as the primary fuel consumption statistic. “If there is a desire to introduce ‘gallons per 100 miles’ estimates,” the agency concluded, “do so in a way that positions it as additional information and use the same font size for presenting the MPG and gallons per 100 miles information.”