DETROIT — Lawsuits and calls for a wider recall of Firestone tires multiplied on Monday as the tire maker's replacement plan failed to calm anger over tire failures and left Ford Motor Co (F.N) privately expressing frustration with its partner of nearly 100 years.

Safety groups called for the expansion of the voluntary recall announced last week to include the 16-inch size of the ATX, ATX II and Wilderness AT tires in addition to all of the 15-inch models. Firestone announced the limited recall in response to customer concern after reports of deteriorating treads and blown tires.

Those fears have led to numerous lawsuits, including class action lawsuits filed on Monday on behalf of tire owners in Florida and Maryland. South Carolina also filed suit seeking a higher priority in the recall plan.

Rating agency Standard & Poor's on Monday lowered its long-term credit rating on Firestone's parent, Bridgestone Corp (5108.T) of Japan, and said further downgrades were possible because of the recall's possible impact.

The U.S. National Highway Traffic Safety Administration (NHTSA) has said it is investigating at least 46 deaths and over 300 complaints about the tires. Those numbers are expected to grow and new data could be released as early as Tuesday.

A majority of the tires being recalled were fitted as standard equipment by Ford to its popular Explorer, the top-selling sport utility vehicle in the United States.

Ford has publicly stood by Firestone, saying it has no intention of switching from Firestone tires on its SUVs and pickups, extending a relationship that dates back to 1906.

"I see no way that we would drop Firestone as a supplier,'' Ken Zino, a spokesman for the No. 2 automaker, told reporters on a late Monday conference call.

However, privately Ford officials have expressed frustration with Firestone's inability to inform the public properly which tires were being affected by the recall and deal with the consumer backlash.

In response, Firestone said it will launch an ad campaign in several major metropolitan daily newspapers on Wednesday to better explain the recall.

Executives at the tire maker insist they are moving as fast as they can, that the recalled tires are the only ones that need to be replaced, and that all their actions have been in the interest of customer safety.

"Maybe 20 years from now, when they're doing studies of this (recall) in Harvard Business School they may say the company should have done this, or might have done that differently,'' Firestone Vice President Christine Karbowiak told Reuters. "But at the time, we were making what we thought were the right judgements.''

Public Citizen, Safetyforum.com and Strategic Safety said the replacement of the larger tires in other countries by Ford combined with 19 U.S. failures the groups had identified provided sufficient grounds for the 16-inch versions to be included.

The groups also questioned the restriction of the recall to those 15-inch Wilderness tires made at Firestone's Decatur, Ill., plant, citing at least three non-Decatur failures.

Firestone's Karbowiak called the groups' charges ''unsubstantiated allegations'' not supported by the company's data that shows all tires other than those recalled are safe.

The safety groups, some of whom work with plaintiffs' attorneys, challenged the phased recall that gives priority to southern states, where most of the incidents have been reported.

"I don't think it's an issue of ambient temperature or weather, I don't think it's a usage issue, a Decatur plant issue -- I think it's a design defect,'' Public Citizen President Joan Claybrook told a news conference.

"There is ample evidence to show that vehicle owners with these other, non-recalled tires may be at substantial risk,'' said Claybrook, a former NHTSA Administrator.

Ford spokesman Jon Harmon said the data showed no reason to expand the recall. "We're looking for solutions for our customers. These people are looking for lawsuits,'' he said.

The number of suits against Ford and Firestone are already estimated at about 100 by plaintiffs' attorneys. In response to the South Carolina lawsuit, Karbowiak said Firestone would ''make happy'' any concerned customers in that state if they visited a Firestone store.

She added the recall is ahead of schedule. Originally slated for completion after next summer, the recall is now on pace to be completed before then thanks to increased Firestone tire output at its plants and use of competitors' tires.

Senate Commerce Committee Chairman and Arizona Republican Sen. John McCain sent a letter on Monday to Transportation Secretary Rodney Slater expressing concern the recall may fail to adequately protect consumers in all states.

Firestone made 14.4 million of the 15-inch tires subject to recall, but only 6.5 million are thought to still be in use.

By either count, it is the second-largest tire recall in U.S. history since Firestone recalled 14.5 million radial tires in 1978. Adding 16-inch tires could set a new record.

While Ford used a company-owned Cray supercomputer to help Firestone analyze and discover the tires that needed to be recalled, a cause for the problems has yet to be discovered.

Meanwhile, Ford launched its own ad campaign on Friday and Sunday to better inform its customers on what vehicles were affected by the recall.

Zino said the automaker has seen a short-term dip in Explorer sales because of customer concerns and there have been reports of some customers insisting Firestone tires be removed from new Ford SUVs before they will buy them.

Ford truck engineering director Tom Baughman said the automaker is sending a team to the Decatur plant to review operations and production records to help isolate the cause.

"We really believe this is most likely a quality defect based on what we know,'' he said, adding that Ford's lower tire pressure requirements, the automaker's specifications for the