As of Wednesday, $96 million in vouchers had been submitted for reimbursement, the National Highway Traffic Safety Administration said.
U.S. Rep. Candice Miller said that the program's coffers could run dry in "a matter of weeks."
The program is apparently accelerating despite criticism that many drivers with fuel-inefficient cars don't qualify under the strict guidelines.
The stringent requirements have led to confusion and frustration for dealers and customers, leading some critics to question how a program seemingly so well-intentioned got so far off track, CBS News correspondent Jeff Glor reported last week.
Cars and trucks must be 1984 models or newer to be eligible for a trade-in rebate. They must get 18 miles per gallon or less in combined highway/city rating - based on the "Estimated New EPA MPG" ratings available at www.fueleconomy.gov.
So a 1993 Mercury Grand Marquis - a boat of a roadster with a big V-8 engine - would not qualify, because its original, official miles per gallon was 19.