Tachometers measure engine rpm.
Tachographs are what records duty, driving and rest time, the days of flouting these laws are over, you won’t make good money by doing so because any employer worth working for would have you out on your ear as soon as any wilful infringing of the rules was discovered, the reason for this is if your caught flouting the law the vehicle operator is at serious risk of losing their operators license and were you working for somebody willing to allow this then you’d seriously need to consider your choice of employer!