pressure spool assembly 20 this fixed ratio is known when the engine is designed and manufactured therefore the instant rotational speed of the shaft of the high pressure spool assembly 20 n2 speed can be determined based on a calculation of the detected instant rotational speed of the gear 40 and the known fixed ratio this is computed from time to time by the eec and as an output result the instant n2 speed other than the rotational speed of gear 40 is displayed and is used as a primary input variable necessary for the control logic of the gas turbine engine the oil pump mounted solution of the present invention is novel and has several advantages including a novel location within the agb which results in among other things a reduced tolerance stack up and a low differential thermal expansion mismatch between the sensor 46 and the gear tips the above description is meant to be exemplary only and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the invention disclosed for example the present invention can be applied to various types of gas turbine engines other than a turbofan gas engine which is used as an example to illustrate the application of the present invention the oil gear pump incorporating a phonic wheel apparatus can be affixed.