A method of pumping a liquid with a tank truck, power is transferred from a diesel engine through a automatic transmission device to a triplex pump, wherein the triplex pump has a fluid provided to an inlet, an fluid outlet pressure and a fluid outlet flow rate, the diesel engine has a rotational speed, and the automatic transmission has an outlet rotational speed, and has multiple gear ratios is provided. The method comprising determining a delta P by comparing the fluid outlet pressure of the triplex pump to a predetermined pressure, determining a delta F by comparing the fluid outlet flow rate of the triplex pump to a predetermined flow rate, adjusting the rotational speed of the diesel engine to reduce the delta P and delta F, wherein the rotational speed of the diesel engine is held constant once either the delta P or the Delta F is approximately zero.