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.

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.