In all the types I've flown tacho time has always been less than the actual elapsed time, depends on the rpm at which you run the engine and time spent on the ground.
If you use the tach time for your pilot logbooks entries, you could be shorting yourself on your flight hours because tach time can be as much as 20 percent lower than the Hobbs time.
https://doublemaviation.com/hobbs-time-vs-tach-time/