4 Sensor Rear Only Upgrade Kit With Buzzer. The system can be programmed to ignore tow bars or spare wheels. The system is compatible with our visual display types listed in related products below. Can be used as a 2 Sensor system if required. The sensor hole size is 18.8mm in diameter. The visible external diameter of the sensor head is 22.9mm with a depth of 20.1mm and they have detachable cables and a 4 and 10 degree angle of tilt in the box.