Should We Be Computing Gallons Per Mile?

How to compute efficiency more efficiently.
By Alison Lakin
Like so many things in life, we’ve grown accustomed to an oft-used phrase without fully appreciating what it even means. Miles per gallon, or mpg, is well known by car owners throughout America. Literally, it means that your car gets X number of miles out of one gallon of gas. For example, the Cadillac Escalade gets 12 miles per one gallon of gas in the city. We also know that the higher the number, the better it is – we like avoiding a stop at the gas station every few days.

(Courtesy of Duke

But is there a better way to compute our fuel consumption? According to researchers from Duke University, viewing efficiency in miles per gallon terms may actually be giving us inaccurate views on efficiency. We generally think that increases in mpgs are a good thing. However, they aren’t all the same.

Researcher Jack Soll says through Duke University, “What we found is that people tend to rank order things in terms of linear improvement in mpg so the bigger the improvement in mpg that’s what they think is better. But that turns out not to be the case mathematically."

Right now, the way we calculate fuel consumption means that the savings if you jump from a 14 mpg car to a 17 mpg car would seem to be the same as the jump from a 26 mpg car to one that gets 29 mpg. The fact that there's a three mile per gallon difference is all we can really determine.

The distinction ‘miles per gallon’ is indeed misleading, but what is easier to understand, Soll maintains, is gallons per X miles – similar to the system many European countries currently use. “If we stick with the idea that people drive [their] vehicles 100 miles a week we can simply ask how much gas is being used by each vehicle."

Say you’re comparing those two vehicles mentioned earlier, one that gets 14 miles per gallon and another that gets 17 miles per gallon. Over the course of 12,000 miles – the average mileage we drive in one year – scooting around in the 17 mpg vehicle would have saved you about 150 gallons of gas over the 14 mpg car. At $3 a gallon, that’s $450 in your pocket at the end of the year.

The same 3 mpg jump from say 26 mpg to 29 mpg only reduces fuel consumption by an extra 47 gallons and saves you much less money – just $141 in a year.

Or for Soll’s example, a 15 mpg car will use 6.7 gallons after 100 miles and a 20 mpg car will use just five. And a 25 mpg car only uses four. The switch between the first two will yield a savings of almost 2 gallons of gas a week, while the latter two will only change by one.

Despite both oil and fiscal savings becoming smaller as fuel economy numbers grow larger, every little bit does count. The less oil we use, the better. With over 200 million cars in our country, there’s a lot of gas that can be saved if we reduced our consumption by just 2-3 mpg.

Duke University researcher Rick Larrick, who coauthored the study, says, “The key insight is that improving inefficient vehicles that have low mpgs by even low mpg increase saves a lot of gas."

During questioning of test subjects, Soll and Larrick discovered that gallons per X miles helped people understand the changes in fuel-efficiency between one vehicle to another more easily. Considering how important this issue has become recently, it would make sense for the consumer – and higher-level officials – to fully appreciate the level of improved fuel consumption he or she stands to gain when upgrading to a more efficient vehicle.

As Larrick points out, “If consumer reports or the U.S. government just…expressed efficiency in terms of gallons per 10,000 miles it would help people recognize where the gains in efficiency are and moving out of the inefficient cars to the efficient cars would become obvious." 

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