John Elsden relishes the excitement of the mint-condition '65 Mustang convertible he takes out of his garage in eastern Virginia only on sunny days. The sporty coupe with a 289-cubic-inch engine growls, rumbles, vibrates and shakes when he puts his foot on the gas pedal.

But the Mustang, Mr. Elsden said, is not as zippy as his daughter's year-old sports wagon, a Nissan Murano, and accelerates hardly better than his wife's 2001 Jeep Grand Cherokee. Mr. Elsden, a car buff who has owned a variety of muscle cars, said the average sedan today accelerated faster than many cars designed to turn heads in the old days.

For two decades automakers have been developing technology that could make vehicles go farther on a gallon of gasoline. But instead, they have chosen pep and size -- making vehicles like the new Murano accelerate faster than cars like the old Mustang, and making them bigger.

The average vehicle, which 25 years ago accelerated to 60 miles an hour in 14.4 seconds, now does it in 9.9 seconds, a pace once typical only of sporty or luxury cars like Camaros and Jaguars. And vehicle weight now averages about 4,100 pounds, up from about 3,200 in the early 1980's, as many buyers switched to larger, roomier cars or to sport utility vehicles and minivans, and as automakers added safety equipment.

Buyers like the extra zoom and room, but these have come at a cost: average fuel economy has fallen slightly over the last two decades. The government's new standards for light trucks like S.U.V.'s, published yesterday, will require an 8.1 percent increase in miles per gallon over the four model years from 2008 through 2011. [Page C1.]

But over the longer term, significant improvement appears to be lagging. As scientists and engineers look for ways to satisfy the fast-growing demand for energy and to slow climate change, many of them say that fundamental changes are needed in the way fuel is produced and consumed.

They say that ways must be found to replace oil and gas, and to get more energy from coal, which will become increasingly important as an energy source, without adding to carbon dioxide emissions.

And they say that greater fuel efficiency is imperative as the number of motor vehicles, which the Worldwatch Institute says has nearly doubled worldwide in the last two decades, could double again in 15 years. The United States has the highest per capita gasoline consumption of any major country.

Higher mileage would also significantly reduce emissions of global-warming gases that come from burning gasoline. The increase in vehicle acceleration and weight alone has raised emissions of carbon dioxide in the United States by about 5 percent.

If 2005 model vehicles, with their better technology, had the performance and size of those in 1987, they would use only 80 percent of the gasoline they do today, according to the Environmental Protection Agency. That alone would get the country nearly halfway to the goal President Bush set in his State of the Union address: to cut American oil consumption enough to nearly eliminate the need to import from the Middle East.

But because Americans have not insisted on better fuel economy, ''we can take the technology in the cars and turn the knob toward performance,'' said Karl H. Hellman, an automotive development expert who retired from the E.P.A. two years ago.

Improving mileage now would be easy if drivers sacrificed some zip in new cars, he said, ''but in this country, we don't sacrifice for anything.''

Just as significant as vehicle weight is how much faster autos accelerate, even as more driving is done on congested roads. The E.P.A. does not calculate how much of the poorer fuel economy results from acceleration and how much is attributable to weight, but it publishes figures suggesting that the split is about even.

The 2005 Toyota Camry, one of the most popular sedans, accelerates more quickly than some 1975 Pontiac Firebirds. The Chevrolet TrailBlazer, an S.U.V., can reach 60 miles an hour in 8.2 seconds -- about half the time of its 1985 ancestor. The 2006 Cadillac STS-V, which can reach 60 m.p.h. in less than five seconds, is one of many cars that now have more than enough horsepower to pull an 18-wheeler.

Philip H. Kosak of High Point, N.C., owns a 1965 Mustang, a 2005 Mustang, a Chevrolet TrailBlazer S.U.V. and an extended-cab Silverado pickup. The whole country, himself included, is spoiled, he said.

He has thought about buying something smaller and more fuel-efficient, especially with gasoline prices often running near $3 a gallon, but he has driven a pickup for years. ''Once you've graduated to that, it's tough to go back. It's like moving back into a smaller home.''

Mr. Kosak, who owns a company that produces snack food, said he drove 30,000 miles a year. He could save hundreds of gallons of gas a year with something smaller, he said, but even at today's fuel prices he can afford the extra expense and does not want to give up the fun of something that performs well. Mr. Kosak, who is 50, says he is looking forward to a lot of driving just for enjoyment.

But nearly a hundred million households burning an extra few hundred gallons a year add significantly to pollution and a greater reliance on imported oil. That contributes to a larger trade deficit and, at times, the use of American power to protect foreign supplies.

Representative Sherwood Boehlert, Republican of New York, who has pushed for years for higher fuel economy standards, said, ''We can talk all we want about our addiction to oil, but words have to be followed with deeds.''

''It wouldn't be a sacrifice at all. No one really needs that greater power, except specialized vehicles for law enforcement purposes.''

The carmakers say they give customers what they want. ''Performance and safety, those factors are salable into the marketplace,'' said Gerald M. Roussel, manager for energy and environment at the Ford Motor Company's government relations office. And they have done so while reducing emissions of pollutants that cause smog and acid rain, he and others pointed out.

In improving fuel economy, virtually everyone agrees that there is a way only if there is a will. ''There is no shortage of technology,'' said John M. DeCicco, a senior analyst at Environmental Defense, a nonprofit advocacy group.

Technologies already in use include four valves for each cylinder, which allows more power from a smaller, lighter engine, and computer-controlled fuel injection, which increases efficiency and cuts pollution.

More changes are coming. For example, gas-electric hybrids are increasingly common, but have been used in some cases more to make the vehicle accelerate faster than to go farther on a gallon of gasoline.

Indeed, Mark P. Mills, a physicist and energy expert, said that improvements in vehicle efficiency would not reduce consumption because without a significant rise in gasoline prices to encourage buyers to save fuel, most people would simply take advantage of the advances to demand bigger, faster vehicles.

Mr. DeCicco agreed. ''What we're really talking about here is the need to, through a combination of public policy and awareness, use the technologies at hand with a different set of priorities in mind,'' he said. ''Until we do that, all the invention in the world isn't going to make a difference.''

The Energy Challenge

Articles in this series will periodically examine the ways in which the world is, and is not, moving toward a more energyefficient, environmentally benign future.

Graph: ''Extra Zoom And Room''
Fuel economy has declined slightly since the mid-1980's, as automakers have balanced gains in technology with faster acceleration and larger, heavier vehicles, and as consumers have increasingly turned to light trucks, a category that includes S.U.V.'s, vans and pickup trucks.

Graphs track Percent of Total Sales, Fuel Economy, Vehicle Weight, Acceleration Time and Power since 1975. (Source by Environmental Protection Agency)(pg. C4)

Chart/Photos: ''Guess Which Is Faster Off the Line''
The 2005 Camry accelerates more quickly than some 1975 Firebirds.

1975 Pontiac Firebird Trans Am 400
0 to 60 miles an hour in 9.8 seconds

2005 Toyota Camry XLE V6
0 to 60 miles an hour in 8.1 seconds

(Sources by; Toyota)(pg. C4)