Ford Motor and Firestone have made considerable progress in their investigations into the tyre-related crashes that have killed more than 100 Americans, mostly in Ford Explorers, but they remain divided on some basic issues, people close to the inquiries said, The New York Times said on Friday. Ford’s newest finding is that a Firestone factory in Decatur, Illinois followed procedures different from those at other factories in handling rubber and other incoming materials, and these procedures may have allowed the quality of the materials to degrade, a person involved in the inquiry said. The steel-belted radial tyres made in Decatur have had a much higher failure rate than tyres of the same brands made elsewhere, according to both companies.

The Times reported that Sanjay Govindjee, an engineering professor and tyre expert at the University of California at Berkeley, and retained by Firestone to conduct a separate review, said preliminary results from tests conducted for him by Firestone provided little evidence for a possible problem cited in a Firestone preliminary report five weeks ago. During normal driving, the motion of the Explorer does not appear to put unusual stress on its tyres that would cause them to fall apart faster than usual, he said in a telephone interview with the Times.

Govindjee did say, however, that the Explorer does have an uneven weight distribution that puts an extra load on the left rear tyre. A disproportionately large share of the fatal crashes have involved failure of the left rear tyre. He said he had not determined whether the extra load was a factor in the tyre failures.

The Times reported that the two companies have made further progress in isolating two other problems in Firestone’s preliminary report, people close to the companies’ investigations said. These problems are a tendency for cracks to develop in the rubber between the layers of the tread, and problems in the design of the rubber wedges that fill the space between the edges of the steel belts.