Triangle Prius owners are pleased with Toyota's gas-electric hybrid car, but they worry sometimes that they aren't working hard enough to squeeze more miles out of every gallon.

Every crosstown drive is a challenge to boost the numbers -- with a digital readout that keeps score.

Owners fret about the drag of low tire pressure and the energy demand of the air conditioner. Though typically careful drivers, they are tempted by the lust for mileage to tailgate tractor-trailers on the interstate so they can draft like NASCAR racers.

"We have this thing on our dashboard that tells us, every minute, what mileage we're getting," said Barry Freeman, 81, of Chapel Hill, whose readings vary between 46 mpg and 51 mpg. "It makes you think of it -- and you maybe think of it too much."

The Environmental Protection Agency rates Prius as the nation's most fuel-efficient car, at an average estimated 46 mpg. Serious Prius people aim higher.

The push to do better is as much about curbing greenhouse gases, they say, as about cutting the use of gasoline.

"We are trying to limit our footprint on the environment as much as possible," said Sandy Scherer, 42, of Raleigh. Her husband's Prius gets 50 mpg.

Sales are surging for the oblong hatchback. Americans have bought 400,000 since 2000, half of them in the past 18 months. Prius was the nation's ninth-most popular vehicle in May, its best month yet with 24,000 sold -- more than all other hybrids combined.

Barbara A. Walters ditched her "typical mommy van" for a blue Prius early last year.

"There were one or two in the parking lot at work before I got mine," said Walters, 49, who commutes to SAS, the software company in Cary. "Since then, they have been breeding like mad. There are about eight there now."

How a hybrid works

Hybrids augment the gas engine with an electric motor, its battery continually recharged with energy created by the brakes. The electric motor boosts power and reduces fuel consumption, sometimes enabling the engine to cut off altogether.

The EPA rates several hybrids, including Prius, better in city than in highway driving because they're at their best in stop-and-go traffic. That makes Prius a good fit for easy-does-it drivers such as Walters, with her daily round-trip between home and office.

Cruising down Harrison Avenue and Cary Parkway on a weekday afternoon, Walters drifts slowly into a knot of cars at the intersection ahead. The dashboard display indicates that the gas engine has switched off, but her passenger notices only the quiet.

When the light turns green and other drivers move out fast, Walters' foot is soft on the pedal. She accelerates gradually, running on battery power and burning not a drop of petroleum. As the speedometer climbs past 25 mph, the gas engine kicks in quietly.

A new style of driving

The digital readout shows that she has driven 477 miles, averaging 52.4 mpg, since she filled the tank about two weeks ago. When one of her sons borrows the car, he is less frugal with fuel.

"He's just a little bit less patient," Walters said. "I think if you're one of those people who are quick off the dime when the light turns green -- if you floor it -- then you're not going to be happy with your mileage."

She was a patient driver before she bought the Prius, but the dashboard readout has reinforced her good habits.

"My mileage tanks when I drive above 70 mph, so I just don't do that any more," Walters said.

Not everyone is a Prius natural. Many new owners encounter a steep learning curve.

Lew Rentel's new Prius got only 35 mpg. He returned to the dealer to complain.

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