As it turned out, the Explorer became the most popular SUV ever, selling at least 3.5 million vehicles to date. And while Ford says the rate of rollover fatalities in the Explorer has been 26% less than for comparable SUVs, its decision to recommend lower tire pressures could come back to haunt it.

The Ford documents, which have been produced in lawsuits, show that as the launch date for the Explorer fast approached, company engineers were still struggling to verify that the new model would be less tipsy than Ford's Bronco II, the rollover-prone SUV that the Explorer was about to replace.

Ford engineers noted in a May 1987 memo that the Explorer prototype actually was worse than the Bronco II in its stability index--a calculation factoring track width and center of gravity height to predict the rollover risk of vehicles. The memo proposed several design changes, including widening the Explorer's track width.