WASHINGTON

THE “cash for clunkers” program will be back in full swing this weekend after President Obama signed a program extension on Friday. The $2 billion infusion should help dealers sell thousands of cars to customers who can reap a bounty of $3,500 to $4,500 for trading in an old vehicle.

The short-term effect will be obvious, but the program’s longer-term value  either as a method of stimulating the economy or saving energy  is less clear. Economists say that most buyers simply moved up the timing of their purchase, and that the projected gasoline savings are exaggerated because many of the trade-ins were seldom used.

The program’s first $1 billion was considered money well spent, at least by the new car industry, because it lured a quarter-million buyers into dealerships; the vehicles they purchased would go about 60 percent farther on a gallon of gasoline than the ones they turned in.

But measuring the stimulative effect of the program would require employing scientific methods like those used in drug trials, where some participants are given a placebo instead of the drug being tested. The problem, according to market experts, is that nearly everyone who bought new cars would have done so at some point anyway.