Abstract: A drive train for vehicles such as earthmoving machines having a prime mover coupled in driving relation with ground wheels for the machine through a power input member and a power output member. One or more implements are coupled to the power input member. In order to selectively match or mismatch power delivered by the prime mover with varying power requirements for the implements and wheels, a hydraulically operated slipping clutch is arranged between the power input and output members for adjusting power delivery from the prime mover to the output member and wheels with a manual control for regulating hydraulic operation of the slipping clutch. In other embodiments, the power input and output members are portions of a torque converter, a centrifugal valve also being provided to operate the slipping clutch in response to rotational speed of an element in the drive train and under regulation of the manual control.