A tractor has an engine which drives the tractor via a transmission whose operative ratio is controlled electronically. The tractor also has an auxiliary hydraulic fluid supply system with an engine driven hydraulic pump which provides pressurized fluid to one or more auxiliary outlets on the tractor. A driver operated control means is provided for selecting the oil flow rate to be delivered to the outlets, and a control system adjusts the speed of the engine to the minimum required to provide the selected oil flow rate and changes the operative ratio of the tractor transmission so that the speed of the tractor remains substantially unchanged despite any change in engine speed required to deliver the selected oil flow rate. The transmission may be a CVT or powershift transmission.