The burden of $4 gas has forced Americans to cut back on driving, and some are slowing down to squeeze more miles out of every gallon. But that doesn't mean we're ready to slash the legal speed limit.

"People that are concerned about the relationship between driving faster and using more gas have every right to slow down on their own," Chatham County resident Mike Melone said via e-mail to The News & Observer.

Pose the question of lowering the national speed limit to 55 mph, as the nation did in 1974, and you'll tap a range of strong feelings. North Carolinians differ about whether drivers would suffer or benefit from a lower speed limit -- or whether we would just ignore it.

In 1974, with gasoline supplies pinched by an Arab oil embargo, Congress directed the states to set a national speed limit of 55 miles per hour. It was a drastic step. But Americans could see that the 1970s energy crisis was serious.

During the worst of it, motorists had to line up for their turns at the gas pump -- and then only on odd or even days, according to their license plate numbers.

We slowed down and drove less, and we saved an estimated 167,000 barrels of oil a day in the late 1970s -- about 2 percent of our highway use. Traffic deaths decreased, too.

This summer, with Triangle fuel prices hitting records of $4.05 a gallon for gas and $4.80 for diesel, the notion of a national speed limit has been floated again -- but cautiously.

U.S. Rep. Jackie Speier, a Democrat from California, says we should slow down to 65 mph on rural interstates and 60 mph on city freeways. Republican Sen. John Warner of Virginia wants Congress and the Energy Department to figure out where we should set our speed limit to save the most fuel.

The Energy Department now tells drivers that fuel economy falls sharply for most vehicles when we travel faster than 50 or 55 mph.

Boxy trucks and sleek roadsters obey the same laws of physics: As they go faster, they must work harder to push the air out of their way. When we double our speed, we quadruple the energy needed to overcome aerodynamic drag.

Engineers at Oak Ridge National Laboratory tested a variety of cars and light trucks and found that, by slowing down from 70 to 55 mph, the average car reduces fuel consumption by 17 percent. For a driver who pays $4 for regular, that's like getting a rebate of 69 cents on every gallon.

"The amount you save varies from one vehicle to another, but if you talk about lowering your speed from 65 mph to 55, it would be somewhere in the 7 to 10 percent range," said Therese Langer, a transportation expert with the nonprofit American Council for an Energy Efficient Economy.

"And this idea is a better deal for those vehicles that have lower fuel economy. For, say, an unaerodynamic SUV, the savings you get from reducing your speed are more significant."

Truckers are on board

Thousands of truckers aren't waiting for the government to order another slowdown. They're doing the math and setting their own speed limits.

"It's a difficult issue," said Clayton Boyce, a spokesman for the American Trucking Association. "People want to be free to run down the highway at whatever speed they can. But there is a need right now, especially for environmental reasons, to conserve fuel."

To improve fuel economy across entire tractor-trailer fleets, many firms have dialed down the top-speed settings in the computers that control the truck engines.

"A rule of thumb is that every 5 mph costs about one-half mile per gallon in diesel fuel," said Duane Long, chairman of Raleigh-based Longistics. "If a fully loaded truck gets only 6 miles to a gallon and slows down from 75 to 65 mph, that makes a big difference."

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