A drive system for use in utility vehicles, such as agricultural forage harvesters
and combines, includes a hydrostatic transmission having a pump driven by a main
engine of the vehicle and having first and second motors respectively coupled to
front and rear axles of the vehicle by way of first and second differential gear
sets. The pump and one or both of the motors have a variable displacement determined
by positioning respective swash plates through the agency of a lever, in the case
of the pump, and by electronic devices, in the case of the motors, the electronic
devices acting in response to control signals sent by a control arrangement, which
in turn, operates in response to signals produced by pressure sensors associated
with the motors and by signals produced by speed sensors associated with output
shafts of the hydraulic motors. The control arrangement has a memory for storage
of data, such as maximum legal ground speed and the ratio of the speeds of the
axles as results from a given wheel size, and uses this information for controlling
the hydrostatic transmission such that the vehicle does not exceed the speed, and
the front and rear axles are driven so as to produce the same ground speed.