Mileage/Hours accuracy

[t-chuck]

Buses have hour meters and odometers. I assume that these meters go with the bus body and are mechanical devices in the dashboard (assume older models).
My assumptions:
Hours go up when the tachometer is non-zero.
Miles go up when the speedometer is non-zero.
Either meter can get disconnected and not go up when they should.
Either meter (the mechanical numbers) can break and be replaced.
Either meter can roll over. ( a million mile bus would probably be obvious though )

As far as the engine and trans go, neither meter means anything when an engine or trans is replaced.

Newer computer controlled engines also have a hour meter that goes with the engine? or does the hour meter go with the computer?

Is it really possible to know what the miles of a bus are? unless maybe it is directly from a school district with records?

Aside from rust, is there a miles/time that is too high for a bus chasis? or a turning point where the suspension will need total replacement?