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Fuel-Saving Speed

Fuel-Saving Speed

With gas prices soaring, a listener called to ask about the best speed for fuel efficiency.


Transcript

The best speed for your wallet. I'm Bob Hirshon and this is Science Update.

Hoping to save some gas money on a road trip, Science Update Producer-on-leave Corinna Wu, called to ask if there was an ideal speed for gas mileage.

We asked Oak Ridge National Laboratory engineer Brian West. He says that in general, the optimum speed is around 50 to 65 miles per hour. Going slower, your engine runs less efficiently, and going faster, the drag from the wind offsets the engine power. But he cautions that other factors weigh in, and that simple solutions may backfire.

West:

If we just have a knee-jerk reaction and lower speed limits everywhere, we could cause gridlock and then fuel economy would actually go down.

If you’ve got a science question, cut the gridlock and call us at 1-800-WHY-ISIT. If we use it on the show, you’ll get a Science Update mug. I'm Bob Hirshon, for AAAS, the Science Society.


Making Sense of the Research

The search for an optimum (ideal level or amount) is a common mathematical problem in everyday life. For example, you need to take in enough calories every day to fuel your body, but not so much that you gain weight. Everyone has an optimum calorie intake for their body and their activity level.

On a larger scale, there’s a lot of political debate now about optimum security levels. We want enough security to make us feel safe from terrorism, but not so much that we feel that our freedom is stifled. (You’d probably be very safe from terrorism by hiding in a bomb shelter the rest of your life, but would it be worth it?)

Since “security” and “freedom” are broad, subjective ideas, there’s no way to pinpoint exactly what the right balance of security and freedom would be. But this listener’s question is a different story. Here, it’s possible to quantify (express as a number) fuel efficiency (in miles per gallon), measure it for any given speed, and plot it on a graph.

If you look at this graph, you’ll see that it starts off very low, then arches up, goes through a couple of bumps, and peaks at 55 mph, and then declines again. That’s because there are two main factors that impact fuel efficiency: engine efficiency (how effectively your engine converts gas into energy) and drag (the wind resistance on your car). As you go faster, your engine efficiency improves, which helps your fuel efficiency, but your drag increases, which hurts it.

To look at it another way, if you’re running the engine and standing still, you have no drag to deal with, but your engine is working very inefficiently. You can burn up a whole tank of gas without going anywhere! As you pick up speed, your engine starts to work more effectively, but the drag increases. For a while it’s still better to go faster (since it’s better to be moving than standing still, and drag isn’t very significant at slow speeds). But at some point, if you go even one mile per hour faster, the extra drag will outweigh any extra engine efficiency, and your energy efficiency will go down. That point is your optimum speed.

Although the average optimum speed for cars is around 55 to 60 miles per hour, it varies from car to car, since every engine works a little differently, and the exact shape of the car affects how strong the drag is (a big SUV will create more drag than a low-rider sportscar – which is why NASCAR cars are so low to the ground). Other factors also influence fuel economy: for example, under-inflated tires and running the air conditioning both hurt your fuel economy.

Now try and answer these questions:

  1. What is an optimum?
  2. What two factors most influence the optimum speed for fuel efficiency?
  3. Can you think of other real-life situations where you would need to find an optimum?
  4. How would you determine the optimum in that situation?

For Educators

Read more about Fuel-efficient Driving at EarthEasy.com


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