Ford agreed in a letter to NHTSA this month to mail letters to the owners of most 2007-08 model vehicles, reminding them to check their tires and keep them properly inflated. NHTSA said drivers can go to a dealer and get a free inspection of the valves.

“During the investigation, Ford became aware that some tire valves produced during a certain time period may not have received an anti-ozone chemical and, over time, may exhibit surface cracks,” wrote R. A. Nevi, Ford’s assistant director of Global Automotive Safety Compliance in an April 6 letter to NHTSA recently posted on a government Web site. “If the cracks are not identified and the valves are not replaced, the cracks may … result in a slow tire air loss.”

Ford will provide NHTSA with the first four months of repair data resulting from the owner notification. Ford said the mailings will begin in mid-May.

The automaker believes “that Ford tire valve stems do not pose a safety risk,” spokesman Wes Sherwood said.

Customers, he said, should contact their dealers if they have questions. Valve stems are covered under vehicle warranties, Sherwood added.

In October, NHTSA opened an investigation after receiving 37 complaints alleging that snap-in tire valves cracked and leaked air, resulting in flats on 2007 Ford vehicles. In total, it reviewed 320 complaints; there were no deaths or injuries.

Shanghai Baolong Automotive Corp., a subsidiary of Topseal Auto Parts, provides valve stems for Ford as standard equipment on most new vehicles.

More than 6 million valves made by Topseal have been the subject of two safety recalls on aftermarket tires.

But Sherwood has said the stems produced for Ford were made in a different part of the plant and with different materials.

Safety advocates wanted Ford to recall the vehicles.

“I’m glad they are sending the letters, but this is a safety issue and they should have been recalled,” said Sean Kane, president of Safety Research and Strategies Inc., a Massachusetts-based firm that works on auto safety issues.

NHTSA said the rate of leaking valves produced between December 2006 and March 2007 was higher than normal — but less than 1 percent of production. It also noted that a substantial majority of those vehicles are equipped with tire pressure monitoring systems, which warn drivers when tire pressure falls below 25 percent of recommended inflation pressure.

The 2007-08 model owners getting letters are for vehicles built between November 2006 and May 2007 and include the F-150, Escape, Expedition, Focus, Ranger, Edge, Mustang, Explorer Taurus, Freestyle, Lincoln MKZ and MKX, and Mercury version of most models.