Originally Posted by
Fl1ingfrog
The tacho is optimised to a constant 2300rpm over 1 hour (known as a 'tacho hour').
I think you will find that mechanical tachometers are produced with various rpm for time equivalence. Some electronic tachometers accumulate elapsed hours from engine start to engine stop with no option to base hours on rpm.
Garmin G3X Touch, which records engine and airframe hours, allows the installer to define how each time shall be measured. If rpm based is chosen then the rpm for 1 tach hour = 1 elapsed hour can be specified by the installer.
FAA definition -
Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing."
ref 14 CFR 1.1 General definitions.
I agree that it's common in USA to use tach time as time in service but that's not what FAA specifies.