High-end agricultural tractors commonly are equipped with power-shift transmissions or hydromechanical power-split gearboxes to optimize speed, power, and fuel efficiency for the specific task at hand. Tractors smaller than 70 hp, widely used by vegetable and fruit growers or landscapers, often are equipped with either mechanical transmissions or hydrostatic transmissions with medium-pressure axial-piston pumps.
Although the hydrostatic transmissions use a variable-displacement pump, they usually need a second gear ratio to provide a broader speed range. However, this second gear ratio can only be engaged when the tractor is stationary. Because the hydrostatic transmission is always engaged, it must be deactivated to interrupt the torque it transmits before gearbox can be shifted. This may not pose a problem, but users are beginning to expect many high-end features in these smaller tractors. These include continuously variable speed and acceleration throughout the speed range, simple handling with semi- or fully automatic gear shifting, fast shifting during travel, and the greater efficiency of a high-pressure, variable-displacement axial-piston pump and axial-piston motor.