There’s so much talk of miles per gallon (MPG) when it comes to buying a car. But according to researchers Richard Larrick and Jack Soll of Duke University, it’s gallons per mile that we should really be thinking about.
Their research on the “MPG Illusion” appeared in the journal Science. It shows that as the miles per gallon of a vehicle increases, there is a diminishing return in gallons per mile.
You can think about it this way, a car that gets 10 miles per gallon will consume 1,000 gallons over 10,000 miles. Meanwhile, a car that gets 15 miles per gallon will consume 667 gallons over 10,000 miles. That’s a savings of 333 gallons of fuel for just 5 MPG increase in efficiency. Now, compare that to a car that gets 20 miles per gallon and consumes 500 gallons per 10,000 miles. You can see that with that next 5 MPG efficiency increase, you’re only saving an additional 167 gallons of fuel per 10,000 miles.
As Larrick explained to Reuters, “There is a math illusion here. The reality that few people appreciate is that improving fuel efficiency from 10 to 20 miles per gallon is actually a more significant savings than improving from 25 to 50 miles per gallon for the same distance of driving”
The crucial take-away from this research is that the greatest gain in fuel savings comes from replacing the most inefficient vehicles. So, if you have two cars and you’re debating which one should go, opting to shed the gas guzzler may make the most sense even if you replace with a vehicle that is only mildly more gas efficient.
Larrick and his team at Duke argue that consumers would be better served by a gallons per mile metric when choosing which car to purchase. They stumbled upon this realization while researching the true fuel efficiency between different cars in a car sharing scheme. They realized that people often mistakenly believe that a 10 mpg increase in fuel efficiency is always equal to the same amount of incremental fuel savings.
But as Larrick explained to The Guardian, replacing a car that gets 10 mpg with one that gets 11 mpg provides the same amount of fuel savings as upgrading from 33 mpg to 50 mpg!
In their research, most people were fooled by the more common MPG information, but when Larrick and his team replaced MPG with gallons per mile, consumers always correctly chose the greener car.
At Sustainable America, fuel savings and fuel efficiency are core to our mission of reducing America’s dependence on oil for transportation. Helping consumers clearly and correctly understand the choices they are making is a critical component to achieving this goal. The small shift to a gallons per mile metric is just one way that we can all help to make this a more resilient and sustainable America.