The National Highway Traffic Safety Administration is investigating nearly 1.3 million Ford F-150 pickup trucks in the United States over reports of an unexpected gear downshift accompanied by a temporary rear wheel lock-up.

The NHTSA is probing claims that some of the trucks suddenly dropped into first gear—to the surprise of the drivers—regardless of how fast they were travelling.

According to the complaints, Ford drivers reported that their F-150s unexpectedly downshifted to lower gears while traveling at highway speeds, followed by rapid deceleration, wit