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NEW YORK (CNNMoney.com) -- Some car shoppers are finding that their trade-in vehicles, which qualified for a Cash for Clunkers rebate last week, don't this week because of changes in the EPA's fuel economy ratings.

In some cases, car buyers say, dealers are backing out of sales they've already made because the EPA changed the fuel economy figures on their trade-in.

"My wife just received a call from the sales manager saying that our clunker doesn't qualify anymore, and that we could either pay the extra $4,500 or return the new car (and get our old car back)," Greg Straka wrote Tuesday on a message board at the Edmunds.com automotive Web site.

He had signed a document agreeing to provide additional documentation needed to process his trade-in, but had not done so yet, Straka wrote.

He had made the deal for his new car last Saturday, the day after program rules were supposed to have been finalized, Straka wrote in an e-mail to CNNMoney.com. But the fuel economy information on the car apparently changed the next day, he said.

Straka declined to name the dealership or the specific car models involved in the disputed transaction.

Another car shopper e-mailed CNNMoney.com saying he went to the Environmental Protection Agency's fueleconomy.gov Web site on Saturday to double-check the fuel economy rating for his 1987 Mercury Grand Marquis. When he had visited previously, the car's combined city and highway fuel economy was rated at 18 miles per gallon, making it eligible for the program.

But on Saturday, he found something different: The fuel economy for his car had been raised to 19 mpg -- one mile per gallon over the maximum fuel-efficiency allowed under the Car Allowance Rebate System (aka Cash for Clunkers). As a result, he became ineligible for a trade-in credit worth up to $4,500.

Even though the program's basic requirements have been known since it was created by Congress earlier this year, Cash for Clunkers didn't become official until Friday. So as part of the official launch, the EPA conducted "quality assurance and quality control effort regarding fuel economy calculations on more than 30,000 vehicle model types spanning the past 25 years," according to an e-mail sent by EPA spokesman Dale Kemery.

As a result, 86 car models became newly eligible for the program. However, 78 models became ineligible, EPA spokeswoman Cathy Milbourne said in a statement released Tuesday night.

The EPA completed its changes by the time the rules were finalized on Friday, Milbourne said.

The updates, which involved calculating average fuel economy out to four decimal places, were required by the legislation, she said.

Car shoppers have been posting comments on various Internet message boards, including several at the automotive Web site Edmunds.com, describing their frustration with the changes.

The owners of a 1993 Camry V6 wagon, a 1995 Saab 900S and a 1988 Toyota 4Runner all describe their vehicles becoming suddenly ineligible for the program around the time the rules officially went into effect.

Consumers who believe their eligibility may have been hurt by EPA's changes should contact the National Highway Traffic Safety Administration, which administers Cash For Clunkers.

"They should call our attention to it," said Rae Tyson, spokesman for the NHTSA. He did not promise, however, that the agency would bend the rules.

Cash for Clunkers benefits are retroactive to July 1, so dealers have been able to make deals since that date. But they had to wait until Friday to file for their reimbursement checks.

Hyundai Motor America has been helping its dealers close Cash for Clunkers deals since July 1 by providing them with cash advances equal to the expected CARS rebate, a Hyundai spokesman said. As of Friday, about 14% of Hyundai sales were Cash for Clunker deals, according to the automaker. Last month, Hyundai sold about 38,000 cars.

It is unclear if any of Hyundai's deals so far were made ineligible by the changes, said Hyundai spokesman Phil Leinert, or how the automaker might deal with situations in which a car's eligibility changed.

Brian Benstock, owner of Paragon Honda in New York City, said he's done 33 Cash for Clunkers deals since July 1. So far, he said, he has had no problems because of altered fuel economy numbers.

"Fortunately, most of our customers weren't on the edge," he said. The average fuel economy of his Clunker trade-ins has been about 16 mpg, he said.

Dealers who entered into agreements with customers before last Friday, based on a fuel economy figure listed at fueleconomy.gov, have only themselves to blame, said Tyson.

"We made it very clear that they would be holding themselves out to some risk if they consummate a deal before the program officially begins," Tyson said.