Professors Richard Larrick and Jack Soll at Duke University assert in their article in Science on June 20 2008 that gallons per mile has more value than miles per gallon.

Jack Soll and Richard Larrick

I agree, as I’ve figured that way and look to what it costs to get the miles we need traveled. Miles per gallon just confuses things when calculating. But in fairness, the MPG is useful, you can measure and judge comparables with the number and make far better decisions. Still, on the other hand, is the raw objectivity of cost per distance or for comparison, gallons per mile.

An example the university press release writer uses is comparing the gains over 10,000 miles. Improving a gas-guzzler getting 18 up to 28 over the 10K miles saves 198 gallons. Going up from 34 to 50 mpg only gains 94 gallons back. Now we don’t want to require reduced driving distances, but the incentive to glare at the guzzler owner is worthwhile. While we intuitively get this, the hard numbers are kind of revealing, aren’t they?

The professors had tested peoples impressions and most people thought that that spread between 34 and 50 being 16 would save better than the difference of 10 from the 18 to 28 sample. But we overlook that the 34 mpg is already holding a 1.88 times advantage over the 18 mpg, thus our baselines are being overlooked for the start of linear analysis.

In fairness to all of us, keeping a calculus formula ready to make comparisons isn’t something we can expect of many people. As Professor Larrick says, “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.” Professor Soll explains, replacing a large vehicle that gets 10 mpg with one that gets 20 mpg reduces gas use per 100 miles from 10 gallons to five, a 5-gallon savings. Replacing a small vehicle that gets 25 mpg with one that gets 50 mpg reduces gas use per 100 miles from 4 gallons to 2, a saving of only 2 gallons. Point made – conclusively.

The professors are recommending that consumer publications and car manufacturers list efficiency in term of gallons per 10,000 miles driven. Larrick offers, “This measure makes it easy to see how much gas one might use in a given year of driving and how much gas, and money, can be saved by opting for a car with greater efficiency.”

Here is the short cut – divide the vehicles mpg number into 100 and you have gallons per hundred. Then simply multiply as needed such as 10, 100 or 1000. Thus a 25 mpg car\100 = 4 or 40 gallons per 1K or 48 gallons per 1,200 miles per month.

The press release also includes a couple other bits of help you might want to have in your inventory. “The greatest fuel savings come from improving the efficiency of the less efficient car.” Another is “Driving the most efficient car available.” And “There are significant savings to be had by improving efficiency by even 2 or 3 miles per gallon on inefficient cars, but because we communicate in miles per gallon, that savings is not immediately evident to consumers.” Aw, it is for popular consumption.

Well done professors. Its good to see that Science magazine has picked up their paper. This is a concept that is used elsewhere around the world and deserves more attention in the U.S. That’s 100/mpg = GPM.

That might help get back some of the miles we’ve been not driving lately.


7 Comments so far

  1. 88 » Blog Archive » A Better Kind Of Number For Decisions on July 2, 2008 8:15 AM

    […] A Better Kind Of Number For Decisions But we overlook that the 34 mpg is already holding a 1.88 times advantage over the 18 mpg, thus our baselines are being overlooked for the… […]

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