US drivers can’t understand fuel efficiency - at least when it is measured in miles per gallon (MPG). When I was growing up in the early 1970s, MPG became a national buzzword as gasoline prices soared.
But when fuel economy figures are presented in gallons per mile (GPM), more drivers were able to compare fuel efficiency between vehicles, partly because the GPM figure can be directly multiplied by the pump price. An owner can calculate the total gallons she will use in a week, month or year, along with the cost.
It’s a handy metric to remember, especially as some drivers wrap their cars with advertising decals to subsidize their commute.
If you’ve filled up your tank, and you know how many miles you’ve driven since the last time you added fuel, just divide the gallons purchased by miles driven to obtain GPM.
To make the GPM figure more understandable, researchers Richard Larrick and Jack Soll presented the figures as gallons per 100 miles (GPCM).
To calculate GPCM, multiply the GPM figure by 100, or divide 10000 by MPG.
The chart below compares MPG (left axis and the blue line) against GPCM (bottom axis and the red line). To use the chart, just find the figure you want to compare, and trace a vertical line up or down to find the conversion:
- A car that gets 10 MPG uses 10 gallons to drive 100 miles.
- A more efficient car that gets 50 MPG only needs 2 gallons to drive 100 miles.
As I was writing this article, I was pleased to discover that Rich Larrick and I both graduated from the College of William and Mary in Virginia in 1986.
See Reuters, the New York Times, Consumerist, the MPG Illusion site at the Fuqua School of Business, and the original article in Science.
Updated 23 June 2008 738 HST: Rick sent me these comments about my blog post:
“GPM is useful when deciding about buying a new car, deciding between cars, etc. It guarantees that people see that improving from 10 to 11 MPG, 16 to 20 MPG, and 33 to 50 MPG all save the same amount of gas over some distance — 1 gallon over a 100 miles or 100 gallons over 10,000 miles. Without GPM, people expect larger linear improvements in MPG to yield more savings (that’s the illusion).”
