A square baler operable to be powered by a prime mover, said square baler comprising: a chassis presenting a baling chamber in which material is compacted and baled; a plunger assembly including a reciprocating plunger head slidably mounted relative to the chassis and operable to reciprocate within the baling chamber to apply a compressive force against the material; and characterised in that the square baler comprises a variable speed transmission including drive and driven components (94,96) and an endless drive element that drivingly interconnects the components, said driven component being drivingly connected to the plunger head, said drive component operable to be driven by the prime mover at a drive input speed, and at least one of said components has an adjustable operating diameter so that the driven component has a variable output speed wherein said at least one component includes a pair of opposed cones (100,102) that are configured to receive the drive element therebetween, and said cones (100,102) being configured to be shifted relative to one another to define the adjustable operating diameter, said cones (100,102) cooperatively presenting a first hydraulic chamber and a second hydraulic chamber configured to receive a pressurized hydraulic fluid, with the addition and removal of hydraulic fluid between the first hydraulic chamber and the second hydraulic chamber shifting the cones (100,102) relative to one another to vary the adjustable operating diameter, said cones (100,102) being configured to be shifted into and out of an unpressurized condition where hydraulic pressure is removed from the first hydraulic chamber and hydraulic pressure is provided to the second hydraulic chamber, and said transmission having an adjustable speed ratio of the output speed to the input speed, and said speed ratio being less than 1 in the unpressurized condition. a chassis presenting a baling chamber in which material is compacted and baled; a plunger assembly including a reciprocating plunger head slidably mounted relative to the chassis and operable to reciprocate within the baling chamber to apply a compressive force against the material; and characterised in that the square baler comprises a variable speed transmission including drive and driven components (94,96) and an endless drive element that drivingly interconnects the components, said driven component being drivingly connected to the plunger head, said drive component operable to be driven by the prime mover at a drive input speed, and at least one of said components has an adjustable operating diameter so that the driven component has a variable output speed wherein said at least one component includes a pair of opposed cones (100,102) that are configured to receive the drive element therebetween, and said cones (100,102) being configured to be shifted relative to one another to define the adjustable operating diameter, said cones (100,102) cooperatively presenting a first hydraulic chamber and a second hydraulic chamber configured to receive a pressurized hydraulic fluid, with the addition and removal of hydraulic fluid between the first hydraulic chamber and the second hydraulic chamber shifting the cones (100,102) relative to one another to vary the adjustable operating diameter, said cones (100,102) being configured to be shifted into and out of an unpressurized condition where hydraulic pressure is removed from the first hydraulic chamber and hydraulic pressure is provided to the second hydraulic chamber, and said transmission having an adjustable speed ratio of the output speed to the input speed, and said speed ratio being less than 1 in the unpressurized condition.