A tachograph combines the jobs of a clock and a speedometer! A tachograph records the truck's speed and the length of time that it is moving or still. Analog tachographs record the driverís periods of duty on a waxed paper disc which constantly turns throughout the work day. The marker moves nearer to or further from the center according to the driving speed. An entire rotation takes 24 hours.
Analog tachographs are vulnerable to tampering and are being replaced by digital tachographs which record data on smart cards.