An off axis geartooth sensor is provided with a center line of the sensor disposed along a line which is not coincident with the center of rotation of the rotatable member to be sensed. Instead, the center line of the housing in which a Hall Effect element is disposed is positioned at a perpendicular distance from the center of rotation of the rotatable member which is determined as the function of a range of dimensions which define the allowable gap between the Hall Effect element and a surface of the rotatable member. The perpendicular distance between the center of rotation and the line along which the proximity sensor is disposed is mathematically determined as a function of the distances between the center of rotation of the rotatable member and the minimum and maximum possible locations of the Hall Effect element along with the angular distance between those two dimensions. The present invention provides a geartooth sensor which is significantly immune to changes in the gap between a Hall Effect element and the surface of a rotatable member which is being sensed. The rotatable member is provided with a least one discontinuity in its surface which causes changes in the magnetic field surrounding a Hall Effect element and, as a result, provides a signal from the Hall Effect element to indicate the proximity of the discontinuity to the Hall Effect element.