A high speed and high force magnetostrictive actuator is the preferred source of continuously controllable motion for the hydromechanical portion of a diesel fuel injector. The actuator converts continuously variable voltage and current into continuously variable force and displacement. A magnetostrictive actuator advances the state of the art of fuel injection by exerting continuously variable control through-out each injection event, including very fast transients free of overshoot or ringing. From rest, magnetostrictive fuel injector actuators have been tested to extend to their full distance of tens of micrometers without ringing and return to their rest position at near zero velocity. Complete cycles, from rest to rest, can occur in well under two hundred microseconds. A method of detecting fuel pressure takes advantage of the continuous variability in certain properties of the magnetostrictive alloy.