Tach time only works for group operated aircraft.
For Commercial operations (pay for hire) hobbs is the way to go.
Hobbs time is equal to clock time while tacho time is not.
Tach time varies with engine load.
You pay for the time you have the aircraft running under it's own power under your control, it's that simple.
You don't complain about being stuck in traffic with a rental car do you?
You bring it back 2 hrs late, you pay for another half day.
That's their rules.
For economical flying, try this:
MASTER SWITCH ................ OFF
Right, no transponder, no radio, no Nav...and a tacho time higher then Hobbs...big
FLAG