-- The nation's top auto safety regulator will ask automakers to disclose whether they allow fleet customers to opt out of buying side airbags or other safety equipment.

Starting in the 2011 model year, the National Highway Traffic Safety Administration "will begin requesting that vehicle manufacturers specifically state whether fleet models have different safety equipment from those sold at dealerships. Any differences will be noted in the ratings information provided," the agency's acting chief, Ron Medford, wrote in a letter announcing the directive.

NHTSA said in the letter that it had found data provided by General Motors Co. for its 2006-2008 Chevrolet Impala and 2008-2009 Chevrolet Cobalt and some Buick LaCrosse models "misleading" because it listed side airbags as standard equipment.

GM had allowed rental car companies and other fleet purchasers to save $145 per vehicle by opting not to purchase side airbags on tens of thousands of 2006-09 models, a policy safety advocates questioned.

"The agency reviewed side airbag data provided by GM for the models in question and found the information provided is misleading," Medford wrote in a Sept. 4 letter. GM spokeswoman Janine Fruehan said Monday the automaker agreed with the government's action.

"We understand and support NHTSA's action," she said.

NHTSA has revised its safercar.gov Web site to reflect that the models had "optional" and not standard airbags and also revised its crash test ratings.

Enterprise Rent-a-Car has acknowledged buying 66,000 Impalas that didn't have side airbags and then selling some as used cars without disclosing they didn't have side airbags. In total, GM may have sold more than 200,000 vehicles without side airbags.

GM said earlier this month it will not allow fleet buyers to skip buying side airbags on 2010 models.

Many of those used vehicles have ended up for sale and have been erroneously marketed as having standard air bags. GM has vowed to take steps to try to prevent dealers and others from improperly advertising those vehicles.

Automakers in 2003 signed a voluntary agreement to improve vehicle compatibility in front- and side-impact crashes and make side curtain airbags standard in all retail sales starting in the 2010 model year for vehicles weighing 8,500 pounds or less.

Brian Latouf, director of GM's Global Structure & Safety Integration Center, said the company wouldn't allow the airbags to be deleted from the list of features available when they are bought by fleet buyers. He also said that vehicles without side airbags are clearly marked in the owner's manual.

But in an interview, he acknowledged that some GM-certified used vehicles without side airbags were being advertised as having the safety feature.

Medford's letter went to Sean Kane, head of Safety Research & Strategies, an auto safety research firm that often works with plaintiffs' attorneys.

He had wrote Medford and GM President and CEO Fritz Henderson urging both to take more steps to disclose that many vehicles billed as having standard side airbags were sold without them.

GM defended the practice of allowing fleet customers to delete features upon order, saying other similar vehicles from other manufacturers didn't have side airbags.

GM also has taken other steps, including:

• Modified its GM Certified Web site so that when a dealer prints the window sticker for one of these vehicles, the side airbag feature will no longer be included automatically in the list of standard features for the vehicle.

• Contacted AutoTrader.com, Cars.com and other major sellers and advised them that side airbags were not included as standard equipment in these certified used fleet vehicles.

• GM also called dealers that had certified used car inventory with the side airbag option deleted so they can remove the window label and replace it with correct information.