In some implementations, a powertrain for powering a fluid pump may include a power source configured to rotate a first drive shaft at an approximately constant speed. The powertrain may include a mechanical transmission. The mechanical transmission may include one or more gearboxes associated with a set of fixed gear ratios, a second drive shaft coupled to the first drive shaft, and a third drive shaft coupled to an input drive shaft of the fluid pump and configured to rotate the input drive shaft at variable speeds or with variable torque based on using different gear ratios from the set of fixed gear ratios. The powertrain may include a torque converter disposed between the first drive shaft and the second drive shaft to enable the first drive shaft to be coupled to the second drive shaft.