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Treebeard's Stumper Answer
11 May 2001

Go The Extra Mile

You're driving a long distance, but you're running low on gas. You could drive slower, but that takes more time and might use more gas. Driving faster will get you there quicker, but doesn't that use even more gas overall? What's your best strategy to get the most miles per gallon of your remaining fuel? Is there a sweet spot for your speed that gives the best fuel efficiency? With local gas prices now above two dollars per gallon, and continuing power shortages in California, and a controversial new national energy policy soon to be announced, this question is an interesting parable.

Local gas prices are up, and so is the average size of the cars I see at my rural school. Conservation means doing more with what we have, not necessarily making do with less. My stumper doesn't depend on what car you drive, though maybe it should. What's the best way to go the extra mile with the limited fuel you already have in the tank? Do we really need a new national energy policy that puts increased production above efficiency?


A ping-pong ball is too light to throw very far, and a shot-put ball is too heavy, but a baseball is just right. Gas mileage works the same way. We get the worst mileage idling at rest, and driving slowly is not much better. Mileage improves with speed up to a point, but air resistance increases with speed squared. Driving twice as fast has four times the drag. At some point aerodynamic drag catches up, and gas mileage declines. The most efficient speed for most cars is between 40 and 60 mph, near the low end of your highest gear. How we personally decide to use our fuel will make a real difference in future energy policy.

Notes:

I'm way too busy at the end of my Dunn Middle School year to give proper attention to this stumper right now. I'll get back to it as soon as I can after our June 1 graduation. But here's a start...

September Update: I never did get back to this stumper over the summer, and now I'm too busy with the start of school... D'oh!

Julie and I floated our own raft down the Green River through Desolation and Gray Canyons in Utah this summer. There are layers of coal along the river, and I got some nice samples. But what a cost to mine coal in this wild country! Better to use less, so we don't need it.

I just bought a new Ford Ranger truck to replace my 15 year old Chevy (pre-Geo) Spectrum that failed its smog check as a "gross polluter" after 150,000 miles, with bad rings and valves. My new truck averages 26 mpg after two tanks of gas. OK, not great. Conservation acquires a new meaning when your money is on the table! But conservation really did help the California energy crises, and I'm sure it will help with gas shortages. This is classic economics. When it costs more, we use less!


Captain Treebeard on the Green River.

"I felt a great disturbance in the force, Luke..."

Often when I have a stumper on my mind, I suddenly find hints everywhere I look. This week, the news is filled with talk about gas mileage and energy policy. The new National Energy Policy report was released on Thursday, May 17. I haven't read this big report yet, but President Bush's press conference (11 May 2001) is telling. By my count, our President mentions tax relief 14 times, conservation 4 times, and technology just twice. A tax refund to cover high gas prices sounds like an industry subsidy to me. Administration connections with the oil industry don't help. Many citizens are upset by these proposals, and I think President Bush himself (18 May 2001) offers good advice that we should act on:

When you hear these folks, it doesn't matter what side of the debate they're on, who are willing to kind of castigate somebody who may have a good idea, stand up and let them have it.

On the same day, the Department of Transportation released a report that the overall fuel economy of new cars and trucks sold in America has dropped back to the lowest average gas mileage since 1980. The blame has been placed on trucks and SUVs, including the new seven-ton Ford Excursion that averages just 10 mpg. How we drive is important, but what we drive makes a bigger difference. Choosing to walk or bike or carpool is best of all. I value my personal right to be stupid, but I'm concerned about the long-term cost of encouraging production over conservation.

Here are a few links to the physics of gas mileage. Is it velocity squared or velocity cubed? (Hint...)

Here are a few links to the political discussion:

Here are some practical links:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org