than tires that operated in northern climates. Tires in warmer climates also showed higher levels of cure and had somewhat lower peel strength. The role of air temperature would explain why geographic regions with higher air temperatures had the highest tire failure rates. These results held up even for tires that had zero percent tread wear. Higher vehicle loads increased belt crack growth rates. Of the four factors involved in the growth of cracks in the tires, vehicle load was the single most important factor. The Explorer weighs more than the Ford Ranger and has a higher Gross Vehicle Weight Rating (GVWR) while also having a lower recommended tire pressure. This difference in weight and tire pressure explains why Ford Explorers had far more tire failures than Ford Rangers even though they used the same tire models. Despite these differences, investigators were concerned that Ford Rangers would also see an increase in tire failures if those tires were not recalled. Tire design Before May 1998, the belt wedge gauge for ATX and Wilderness AT tires was less than for comparable tires from other manufacturers. The wedge gauge on completed tires was also less than Firestone's minimum standards. The wedge on these tires was not large enough to resist crack formation and growth. The design of the belt wedge changed in May 1998 making it larger and comparable in size to tires manufactured by other companies. On Wilderness AT tires the inter-belt gauge, or a strip of rubber between the two steel belts, was often thinner than Firestone's minimum specification and the design was thinner than comparable tires from other manufacturers. The design of the shoulder pocket, a "cavity between the heavy ribs, or lugs, along the shoulders of the tire.", on the Wilderness AT tires could lead to high stresses along the tire edge allowing cracks to form in the belt wedge. Peel adhesion tests, where a strip of the second belt was pulled off the body of the tire, showed that Wilderness AT tires were weaker than similar tire models manufactured by other companies. Wilderness AT tires were comparable in strength to tires manufactured at the Decatur, IL factory, regardless of which factory the Wilderness AT were manufactured in. Wilderness AT tires had similar shearography results regardless of which factory produced them. Cracks and separations were more frequent than similar tires produced by other manufacturers. In 2000 Firestone added