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Can you drive 55?

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With the national average gas price breaking $4 a gallon and looming fears of peak oil and catastrophic climate change, some advocates are calling for the American driving public to take its collective foot off the gas.

Shortly before Memorial Day weekend, the Sierra Club exhorted drivers to slow down with its I Can Drive 55 pledge to obey the speed limit. There's also drive55.org, a site launched in 2002 by ecopreneur Tim Castleman that has been getting more attention lately. Mr. Castleman's site includes statistics indicating that easing up on the gas saves money, the planet, and lives. His site also sells bumper stickers with messages ranging from "55 for peace" and "55: support the troops."

Why does driving more slowly save gas? It's a matter of drag. Your wind resistance increases as a square of your speed. So at 80 miles per hour, the wind resistance is more than twice what it would be at 55 miles per hour, because 80 squared (6,400) is more than twice 55 squared (3,025). But at 80, you're only traveling 45 percent faster than you are at 55.

Take it from no less an authority than the Car Talk guys, whose website has some excellent resources on fuel economy:

Wind resistance increases dramatically with speed. That's why aerodynamics have become so important in the last 15 years, and why all of our cars now look like jelly beans.
How much does it matter? Consider this: for every ten miles per hour you floor it, you lose as much as 15% in fuel economy. What's that mean for your retirement account? For every 1,000 miles you drive, figuring gas at $2.50 a gallon and 25 MPG fuel efficiency, you'd save as much as $15 if you drove 10 mph slower. Over the course of the year, that's enough to buy that nuclear powered, stainless steel cappuccino machine you've been coveting – and have enough left over for beans, too.
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