Speeders wasting big bucks on gas

Easing off the accelerator could save 15 cents a gallon, Energy Department says

The Cost Of Energy

August 26, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

If you could save 15 cents a gallon by filling up at a gas station that's a bit out of your way, would you do it? In these days of $2.69-a-gallon gasoline, most people probably would.

But how many drivers would cut back their highway speeds by 5 mph to save the same amount?

They could, according to the U.S. Department of Energy.

For every 5 mph a motorist drives above 60 mph, the gas wasted is the equivalent of shelling out an extra 15 cents a gallon. So streaking along at 75 mph is like spending an extra 45 cents a gallon.

(And these Energy Department numbers were based on gas at $2.20 per gallon, a price that now seems, well, quaint. The savings are better today.)

So slow down. That's just one in a long list of recommendations that government and automotive experts suggest to ease the pain of rising gasoline prices.

Most involve a paltry expense, or none. But they often require renunciation of bad driving habits. Backing off the accelerator on the highway is one of the easiest.

The government says driving faster than about 60 mph sends your car's fuel efficiency plummeting by 7 percent to 23 percent. By the time you're doing 75 mph, you're getting about the same gas mileage as you would at 25 mph - in other words, you're getting city-driving mileage at highway speeds.

Of course, at 75 you're also breaking the law and risking higher-energy crashes with a greater likelihood of getting yourself and people around you killed.

Scientifically, there are two villains at work here. One is mechanical - comprising such things as gear ratios, rolling resistance and friction in the drive train.

"Most cars are designed to a sweet spot of 40-60 mph," a speed where the engine and transmission can run at their most efficient for long periods of time, said Pat Hammett, a scientist at the University of Michigan's Transportation Research Institute.

"When you start to get above that, you start to actually overwork the engine," he said.

The second villain is air - or, more precisely, wind resistance.

Above 40 mph, aerodynamics become the most critical issue by far, said Jewel B. Barlow, director of the Glenn L. Martin Wind Tunnel at the University of Maryland College Park. That's because wind drag increases with the square of the speed.

"So if you double the speed, wind resistance goes up by a factor of four," he said. At 70 mph, aerodynamic drag is nearly twice as great as it was at 50 mph.

That means the gallons you burn strictly to overcome air resistance double between 50 mph and 70 mph, he said. "And the faster you go, the more dominant wind resistance is."

That's why NASCAR racing engines - probably the most efficient there are - get only 3 miles per gallon. "The reason is, they're driving 200 mph," Barlow said.

Weather, too, can cost you. For a driver going 60 mph, a 5 mph headwind instead of a 5 mph tailwind can impose a 40 percent penalty in wind resistance, and the fuel needed to overcome it. In a perfect world, we would always drive down-wind.

Automakers have spent huge sums to redesign vehicles with smoothly contoured, "slippery" lines that are better at shrugging the wind aside.

"It's why the Taurus became a rounded automobile, and why everybody had to follow suit," Barlow said.

Ford introduced the "jellybean" shape in its 1986 Taurus. By 1992, it was America's top seller. Sharp, aerodynamically "sticky" corners were suddenly out. The auto industry's designers have since reduced average aerodynamic resistance by about 40 percent.

Today's sedans have "drag coefficients" - an index for comparing the slipperiness of car shapes - of about 0.3, compared with 0.5 in the 1970s. Sport utility vehicles, with a relatively large frontal profile, have drag coefficients of about 0.4 or more.

"A 10 percent reduction in the drag coefficient gives you a 5 percent improvement in highway fuel consumption," Barlow said.

Truckers have learned the lesson. More aerodynamic shapes and roof and side fairings on truck tractors have cut the 18-wheelers' fuel consumption by 30 percent in the past 25 years, Barlow said.

New technologies can work wonders with gas mileage. For example, GM and Honda are developing vehicles that will shut down four of a car's eight cylinders during steady cruising. That's because the engine's full horsepower is needed only for quick acceleration, not steady speed.

Idling at a red light, of course gets you zero miles per gallon. So some GM pickups and all gasoline-electric hybrids shut off the engine completely when the car is stopped, starting it again instantly when the driver pushes the accelerator.

But the very acts of stopping and starting are also costly, Hammett said. To get started, you have to shift through the gears, running the engine at higher revolutions without getting very far. And the heavier the car, the more gas that takes.

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