hydraulic motor 48 rotates rear driving member 11 it is also optional to provide a vibration means 44 46 on either or both front and rear driving members 5 11 for a double drum compactor we prefer a vibration means 44 on both front and rear driving members 5 11 referring to fig4 and 5 means for controlling the on off point of a vibration means 44 in vehicle 1 will now be described hydraulic motor 40 interacts with a speed reducer 50 having a plurality of gear teeth 52 rotating about axle 7 of drum 5 a sensing device 54 induces a magnetic field through which gear teeth 52 move as each gear tooth 52 and its adjacent gap moves through the magnetic field sensing device 54 detects an intermittent change in magnetic field flux as a plurality of intermittent events herein termed 34 pulses 34 caused by the presence and absence of the metal gear tooth these 34 pulses 34 are transmitted as a plurality of intermittent electrical signals to microcontroller 26 referring to fig6 and 8 an alternate embodiment of means for controlling the on off point of a vibration means 44 in vehicle 1 will now be described an annular pick up ring 70 is mounted adjacent speed reducer 50 on axle 7 for rotating with axle 7 axle 7 is supported by frame member portion 71 as is conventional ring 70 has a plurality of teeth 72 formed on its outside diameter sensing.