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Fuel Efficiency Revisited


by Bill Chameides | July 7th, 2008
posted by Erica Rowell (Editor)

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For many Americans, summer means road trips. But with fuel costs on the rise, whether or not this year’s vacation fits into the budget is up for question. Recent statistics show that Americans are shifting away from larger, gas guzzling vehicles, but a new study published in Science highlights that many of us don’t understand fuel economy basics. How much gas can you save by trading up to or renting a more fuel efficient car? The answer depends on what you’re driving now. Let’s take a look.

Understanding Fuel Economy Differently

Duke researchers Richard Larrick and Jack Soll wanted a better way to understand fuel economy. Their hunch was that consumers didn’t understand that the relationship between a car’s fuel economy and the amount of gas it burned was nonlinear — in other words the amount of gas saved does not increase by a constant amount as fuel economy increases.

Feeling a little confused? Consider this: Suppose you’re contemplating taking a 1,000-mile road trip and gas prices are $4 per gallon:

If you have a 15 miles per gallon (mpg) guzzler, fuel for the trip cost you a whopping $266.67.

Here are the savings you get if you did the same trip with a more fuel-efficient car:
20 mpg – a savings of $67
25 mpg – a savings of $107
30 mpg – a savings of $133
40 mpg – a savings of $167
45 mpg – a savings of $178

The chart below shows the savings for the same 1,000-mile road trip at $4/gallon relative to a car with 25 mpg and 35 mpg, as well as the 15 mpg example described above.

Think ‘Gallons per Mile’

The interesting and possibly surprising feature of these results is that the incremental benefits of a 5-mpg improvement in fuel economy decline as we go to progressively more and more fuel efficient cars. You save $67 by going from 15 to 20 mpg, but an additional 5 mpg improvement only nets you an additional $50 and the next 5 mpg nets even less — $26.

Those of you driving gas guzzlers can save a significant amount of money — and by extension consume less gasoline and emit less greenhouse gas pollution — by opting for a car with just 5 or 10 mpg better fuel economy.

For those of you already driving cars with decent fuel economy (say in the 30 mpg range), it probably makes more sense to keep it rather than to buy a new car with somewhat better fuel economy. The savings will be modest and the amount of energy and global warming pollution you will save by driving the more fuel-efficient car may not be enough to offset that expended in the production of the new car itself.

There is also a clear message here for policymakers. The quickest and most effective way to cut the nation’s consumption of oil and our greenhouse gas emissions from automobiles is to develop policies that get the gas guzzlers off the road.

 

Want to learn more about this nonlinear relationship between fuel usage and miles per gallon? Check out this online quiz by Larrick and Soll.

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