WASHINGTON - The most common deals under the government's $3 billion Cash for Clunkers program, aimed at putting more fuel-efficient cars on the road, replaced old Ford or Chevrolet pickups with new ones that got only marginally better gas mileage, according to an analysis of new federal data by the Associated Press.

The single most common swap -- which occurred more than 8,200 times -- involved Ford 150 pickup owners who took advantage of a government rebate to trade their old trucks for new Ford 150s. They were 17 times more likely to buy a new F150 than, say, a Toyota Prius. The fuel economy for the new trucks ranged from 15 mpg to 17 mpg based on engine size and other factors, an improvement of just 1 mpg to 3 mpg over the clunkers.

Owners of thousands more large old Chevrolet and Dodge pickups bought new Silverado and Ram trucks, also with only barely improved mileage in the middle teens, according to AP's analysis of sales of $15.2 billion worth of vehicles at nearly 19,000 car dealerships in every state. Those deals helped the Ford 150 and Chevy Silverado -- along with Ford's Escape midsize SUV -- climb into the Top 10 most-popular vehicles purchased with the government rebates. The most common truck-for-truck and truck-for-SUV deals totaled at least $911 million.

In scores of deals, the government reported spending a total of $562,500 in rebates for new cars and trucks that got worse or the same mileage as the trade-ins -- in apparent violation of the program's requirements. The government said it is investigating those reports and said in some cases they were probably entered incorrectly by dealers or based on outdated fuel economy figures.

The new data, obtained by the AP under the Freedom of Information Act, include details of 677,081 clunker trade-ins processed by the government through Oct. 16. More than 95,000 of the new vehicles bought under the program -- or about one in seven -- got less than 20 mpg, the data showed.

The new figures, requested four months ago by the AP, represent the first substantial outside accounting of the clunkers program, lauded by the White House and the Transportation Department for improving fuel economy, stimulating sales and taking the dirtiest vehicles off the road. The data show the average fuel economy was 15.8 mpg for the old vehicles and 24.9 for the new ones. But plenty of consumers bought relatively low-mileage trucks and SUVs with the help of government checks.

"If we're looking for the environmental story here, we're going to be disappointed," said Jeremy Anwyl, chief executive at Edmunds.com, an analyst firm. "It might have started out from the perspective of improving the environment, but it got detoured as a way to stimulate the economy."

Popular high-mileage commuter cars including the Toyota Corolla, Honda Civic, Toyota Camry and Ford Focus also were among the Top 10 most popular new vehicles bought under the program, with 105,280 of those models sold for a total of about $2 billion.

The program ran from July 27 to Aug. 25 and generally required that new vehicles get better mileage -- at least 22 mpg for cars and either 15 mpg or 18 mpg for trucks depending on class -- and that trade-ins get no more than 18 mpg. The trade-ins were required to be destroyed in exchange for either $3,500 or $4,500 rebates.

"The value that the customer got for a lot of these vehicles was just a gift, no question," said Scott Pundt, sales vice president for the Dorschel Group of Rochester, N.Y., the No. 4 U.S. dealership under the program.

Some deals raise eyebrows:

• In at least 145 cases, the government reported consumers traded old vehicles that got better than or the same mileage as the vehicle they bought. The government said it was continuing to investigate. A driver in Negaunee, Mich., traded a 1987 Suburban that got 18 mpg for $3,500 toward a new Silverado pickup that got only 15 mpg. An Indianapolis driver traded a 1985 Mercedes 190 that got 27 mpg for $3,500 toward a new Volkswagen Rabbit that got only 24 mpg. "It's possible some quirky deal slipped through the cracks," Anwyl said.

• In at least 15 deals in nine states, owners cashed in old trucks to buy new Hummer H3 SUVs that got only 16 mpg.

• In at least 32 deals, drivers traded older vehicles for new large trucks -- including versions of Toyota Tundras, GMC Sierras, Chevrolet Silverados, Dodge Rams and Ford F150 pickups -- that got only 14 mpg.

• A driver in West, Texas, earned $4,500 in July in exchange for a 1989 Chevrolet Suburban that got 14 mpg and bought a 2009 Suburban that weighed 5,900 pounds and got 16 mpg.