WASHINGTON (AP) — Billed as a way for the government to put more fuel-efficient vehicles on highways, the popular $3 billion Cash for Clunkers program mostly involved swaps of old Ford or Chevrolet pickups for 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 F150 pickup owners who took advantage of a government rebate to trade their old trucks for new Ford F150s. 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 F150 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it is investigating those reports and said in some cases they were probably entered incorrectly by dealers or based on outdated fuel economy figures.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 National Highway Traffic Safety Administration is still reviewing the reports, and any dealers that submitted invalid trade-ins will be directed to return the government rebate, spokesman Eric Bolton said.

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 purchased under the program — or about 1 in 7 — got less than 20 mpg, according to the data.

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.

The overall mileage increases over the clunker fleet represent a decline of 1.87 million tons of carbon dioxide per year, based on families driving an average of 12,000 miles, a yearly savings equivalent to the amount of carbon dioxide spewed in the U.S. in just 2.5 hours.

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 four-week program, with 105,280 of those models sold for a total of about $2 billion.

Bolton, the NHTSA spokesman, said the program “proved to be a win for the economy and the environment” because it helped financially struggling dealerships and auto manufacturers, and because, under the program’s rules, clunkers necessarily were replaced with vehicles that got better mileage.

Chris Moss of Smithtown, N.Y., traded in his 1992 white Ford F150 pickup — “it had 5 million miles on it and needed $50,000 in repairs, if you know what I mean” — for a new Chevrolet Malibu hybrid for his wife. When he drove his old truck to the dealership’s back lot with the rest of the clunkers, “90 percent of what you saw were old 150s and Explorers,” he said. Moss posted a video on YouTube of his old truck’s final day, called “Rust In Peace.”

The $3 billion program, known officially as the Car Allowance Rebate System, 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.

Some deals raise eyebrows:

— In at least 145 cases, mostly involving trucks, the government reported consumers traded old vehicles that got better than or the same mileage as the new vehicle they purchased. The government said it was continuing to investigate. “It’s possible some quirky deal slipped through the cracks,” Anwyl said.

— In at least 15 deals in nine states, owners of large pickups cashed in old trucks for between $3,500 and $4,500 toward 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.

Car-crazy California led clunker sales with more than 76,000 trade-ins, followed by Texas with roughly 43,000 and New York with nearly 37,000. In California, the Honda Civic was the No. 1 new car and no pickups ranked higher than 18th. In New York, the Hyundai Elantra was No. 1.

The clunkers program was very good for Longo Toyota of El Monte, Calif., just east of Los Angeles, which sold more than twice as many vehicles under the program as any other dealership in the country, worth more than $30 million. That sole dealership was responsible for 1,432 sales worth nearly $6 million in clunkers rebates, mostly from its sales of 323 Toyota Camrys, 277 Corollas and 171 Priuses.

“We knew it was just a matter of when, not if, we were going to get paid, so we kept our foot on the gas,” Longo president Tom Rudnai said.

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On the Net:

Cash for Clunkers data: http://www.cars.gov/carsreport