NEW YORK -- Drivers have long known that slowing down on the highway means getting more kilometers to the liter. Now U.S. airlines are trying it, too -- adding a few minutes to flights to save millions of dollars on fuel.

U.S. low-cost carrier Southwest Airlines started flying slower about two months ago, and projects it will save $42 million in fuel this year by extending each flight by one to three minutes.

On one Northwest Airlines flight from Paris to Minneapolis last week alone, flying slower saved 162 gallons, or 613 liters, of fuel, saving the airline $535. It added eight minutes to the flight, extending it to eight hours, 58 minutes.

That meant flying at an average speed of 856 kilometers per hour, down from the usual 872 kilometers per hour.

"It's not a dramatic change," said Dave Fuller, director of flight operations at JetBlue, which began flying slower two years ago.

But the savings add up. JetBlue adds an average of just under two minutes to each flight, and saves about $13.6 million a year in jet fuel. Adding just four minutes to its flights to and from Hawaii saves Northwest Airlines $600,000 a year on those flights alone.

United Airlines has invested in flight planning software that helps pilots choose the best routes and speeds. In some cases, that means planes fly at lower speeds. United estimates that the software will save it $20 million a year.

"What we're doing is flying at a more consistent speed to save fuel," said Megan McCarthy, a United spokeswoman.

United expects to pay $3.31 a gallon for fuel this year -- not much less than what the average U.S. driver pays for a gallon of unleaded at the pump. Southwest, which has an aggressive fuel hedging program, expects to pay about $2.35.

Fliers, already beleaguered by higher fares, more delays and long security lines, may not even notice the extra minutes. The extra flight time is added to published flight schedules or absorbed into the extra time already built into schedules for taxiing and traffic delays.

Across the board, airlines are feeling the pain of higher energy prices. For jet fuel delivered at New York Harbor, the spot price -- airlines pay it when they need more fuel than they have already locked down in a contract -- has jumped 73 percent in the past year, to $3.54 a gallon, according to government data.

Airlines are trying other measures as well to deal with higher fuel costs, including raising fares, adding fuel surcharges to tickets and charging extra for a second checked bag rather than a third.

Not every airline is taking the slowdown approach.

"We have the flying schedule to protect," said John Hotard, a spokesman for American Airlines. He said the carrier does other things to save fuel -- for instance, installing small vertical stabilizers called winglets to the ends of some aircraft wings, which boosts fuel efficiency by improving aerodynamics.

American also tries to keep its planes plugged in to ground-based power and air conditioning for as long as possible to conserve fuel, and pushes air traffic controllers to assign its flights to altitudes where they will have less headwind or greater tailwind. Many other airlines have adopted similar measures.

Slowing flights down is not a magic bullet. It can help airlines conserve fuel, but it can also lead to greater labor and maintenance costs if airline employees work longer hours and planes spend more time in service, said Bob Mann, an independent airline consultant based in Port Washington, New York.

And slowing down to conserve fuel can only be pushed so far: Below a certain speed -- which varies depending on the plane -- an aircraft's fuel usage can actually rise.

Airlines must strike a delicate balance, seeking an aircraft's "sweet spot" on fuel use without slowing down so much that other costs, and flight delays, rise, Mann said: "Everything's a tradeoff."