What is Tacho time?

Tacho (tachometer) time is engine time scaled to rpm, typically used for maintenance scheduling rather than billing.

A tachometer accumulates time at a rate that depends on engine rpm, so an hour at cruise power records differently from an hour at idle. This makes tacho time a good proxy for engine wear, which is why maintenance intervals are often tracked against it.

Hobbs time, by contrast, runs in real time whenever the engine is on, and is the usual basis for hire charges.