For the power trains of 4WD vehicles, the gear ratio of the front differential is not properly set, and therefore, while driving in 4WD mode, twisted torque may occur on the power train in proportion to the difference between the numbers of front and rear tyre revolutions depending on the wear of the rear tyres and the load conditions. With such torque, even as the mode is manually switched from 4WD to 2WD, the driving may remain unchanged. When driving at high speed in such a condition, the front differential may generate heat damaging the oil seal, which could allow oil to leak, in which case the bearing and the gear could burn out and at worst the front wheels could lock causing the vehicle to stall.