Three major rental car companies are questioning the results of a National Highway Traffic Safety Administration (NHTSA) survey that helped lead to the recent introduction of the proposed Safe Rental Car Act.

Enterprise, Hertz, and Avis-Budget have sent a joint letter to the National Highway Traffic Safety Administration stating that the information in the survey "does not accurately reflect the performance of our respective companies in this area."

A day before the letter was sent to NHTSA, U.S. Sen. Charles Schumer said the NHTSA study inspired him in part to announce the Safe Rental Car Act, which would make it illegal for rental car agencies to rent out cars that are subject to a recall notice.

The law stating that auto dealers cannot sell cars that have been recalled does not extend to rental car agencies. Schumer criticized that lack of regulation.

In a study of 10 General Motors and Chrysler recalls launched by NHTSA between June 2006 and July 2010, Enterprise after 90 days had fixed an average of 65 percent of the cars subject to the recall. For Avis/Budget, 53 percent of the cars were fixed within 90 days of the recall. At Hertz, only 34 percent of the recalled cars had been fixed within 90 days.

The agencies said in the letter that the auto manufacturers' data provided to NHTSA was not correct because the information was not up-to-date.

The letter stated that manufacturers rarely have an accurate count of the particular vehicles in a rental car fleet because the vehicles are often sold quickly and the new ownership is sometimes not registered until months later. Therefore, the letter points out, the total numbers of vehicles reportedly owned by the companies while under recall will always be higher than the actual number.

An Enterprise spokesperson said that under current company policy "virtually all" cars under recall are grounded until they're fixed, except for rare exceptions in which the company chooses not to fix certain recalled cars.

The spokesperson said rental car companies receive thousands of recalls issued every year with no differentiation for the severity of the issue. Enterprise bases the rare exceptions in which cars under recall on not grounded on information provided by the car manufacturer. After the survey, Hertz told ABC News it dramatically changed its policy to ground all cars under recall.

The letter also noted that sometimes an inspection or repair of a vehicle under recall goes inadvertently unreported, lowering the number of fixed cars.