CHICAGO - In early May, Browning Mitchell said the last time he bought gas, there was still ice on the roads. By the time he fills up again, he estimated, it could cost $4 a gallon.

Mitchell says he averages 73 miles per gallon in his Toyota Prius thanks to "hypermiling."

Mitchell, of Chicago's Mount Greenwood community, doesn't hide his car in the garage. He drives it at least 38 miles a day to and from his City Hall job as an engineer with the Chicago Fire Department.

He displays no outward sign of miserliness, but getting the industry-leading 60 miles per gallon in the city from his 2004 Prius just isn't good enough for him and a small but growing band of "hypermilers" - motorists committed to squeezing the maximum use out of every drop of gas.

"There's three types of people who buy hybrid vehicles," Mitchell says. "Stingy people, people concerned about the environment and geeks who are into the technology."

Mitchell, who travels across the country in his spare time meeting other Prius owners and helping them modify their cars, is in the last group, although many of his techniques can be adopted by drivers of traditionally engined cars.

He's stuffed foam in his car's grill to keep the engine hot and added a meter to his dashboard that measures how far he's pressed the gas pedal. He keeps his tires inflated high enough above the recommended maximum that he's too embarrassed to admit just how high. He also plugs an engine block heater into the electrical system at his home every night so his car performs efficiently from start up in the morning.

He drives with the windows shut and the air conditioning turned off for months after most of us have it cranked on full, keeps his coat on and the heater off in winter and has made an art of driving without even the electric part of the Prius's hybrid motor.

And since his last trip to the gas station, he's averaging 73 mpg - his best ever.

By the stern standards of hypermilers, though, Mitchell is unextreme, and he in fact prefers the term "eco driving."

His car still has a rear seat, and he's added a subwoofer and amp so that he can listen to loud 70s rock music, a weight addition that would make stricter hypermiling devotees blush.

He once took a longer, slower route across Indiana, to boost his mileage, he admits, but insists, "the real reason was to avoid the tolls."

By contrast, Wayne Gerdes, a pal of Mitchell's who lives in the northern suburbs, won this year's Hybridfest mileage challenge in Wisconsin by averaging 181 mpg in a Honda Insight, gets 59 mpg from a stock Honda Accord, using tricks including turning off the engine and coasting without power-assisted steering or brakes.

Then there's the "different" guy who "fitted a fifth wheel to his Insight" to boost mileage, Mitchell says, adding, "There's some guys who wear ice vests keep the AC off all summer."

A ride with Mitchell reveals useful tips any motorist could use.

By predicting when lights will change and coasting up to stop signs, Mitchell may occasionally enrage "a soccer mom in an SUV" behind him, but he usually doesn't arrive any later than anyone else.

For a Chicago motorist, he's preternaturally calm, too, although figuring whether his aversion to road rage is a consequence of hypermiling or a prerequisite may be an unanswerable chicken-and-egg question.