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Given the option, would you prefer to switch from a vehicle that gets 12 MPG to one that gets 14 MPG or from one that gets 28 MPG to one that gets 40 MPG? You might think the answer here is obvious: the second option, of course. Yet, if your guide in picking is whichever switch grants you the better fuel efficiency, you'd be wrong.
Wait, 12 to 14 MPG is better than 28 to 40 MPG?
That would be because, like most people, you probably consider the amount of gas consumed by a vehicle to decrease as a linear function of its MPG. Well, according to an article in Science written by Richard Larrick and Jack Soll (sub. required), two economists at Duke University's Fuqua School of Business, the relationship is actually curvilinear -- which means people often underestimate the gains that can be achieved by removing the most fuel-inefficient vehicles.
GPM over MPG
Therefore, they argue, it's time to ditch the MPG measure in favor of one that would actually make the gains of better fuel efficiency more transparent: liters per 100 km, or another measure that represents fuel efficiency in terms of the amount of gas consumed for a given distance.
Going back to the aforementioned comparison: While it may seem insignificant, switching from 12 to 14 MPG actually represents a fairly large reduction in gas consumption -- 714 gallons to cover 10,000 miles versus 833 gallons to cover the same distance. This reduction is larger than the one achieved by switching from a vehicle that gets 28 MPG to one that gets 40 MPG.
Translating GPM into better policy
Larrick and Soll go on to argue that, from a policy perspective, it makes better sense to adopt a GPM (gallons per miles) metric and to focus on removing the most fuel-inefficient vehicles from the roads. GPM gives drivers a better representation of how much gas they are consuming on a given trip and, if paired with data showing them how much carbon they are emitting, would do a lot more to help them reduce their fuel consumption and carbon footprint.
Applying this metric across other modes of transportation would provide a universal measure that would help people more effectively compare the energy intensity of their activities. More transparency and clarity will mean more reductions over the long term. Technology Review's Kevin Bullis has more insightful analysis on the issue.
Via ::Science: The MPG Illusion (magazine)