Gallons per mile: Making fuel economy easy to understand

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).”

Tagged as: cars, duke, economy, management, research, USA, virginia, william-and-mary