dublinpilot - what you have described is exactly what I understand by a tacho gauge. My interpretation is that it is just geared from the drive cable and integrates the number of turns of the prop. It is set to increment by one full unit if the prop is run for one hour at its cruise setting (whether that be 2300 rpm or whatever). If you run at 2530 rpm (ie) at 110%, then the meter will show 1.1 hrs after an hours running.
If you want true running time to be measured, then you must use a Hobbs meter or Datcon meter, which is not driven from the propellor - it is 12V powered (see quote below for discussion of this) - these are both digital cumulative clocks that are independent of engine settings.
See the Web link / quote below for a far better explanation!
Info on Engine Instruments
Q. I have a Hobbs meter fitted to my Beech Bonanza as well as the tach time indicator on the RPM indicator. These two always seem to indicate a small difference. Why is this and which one is more reliable?
A. It is difficult to say which of the two indicators is the more accurate one as it is largely dependant on how the Hobbs meter is activated.
Firstly, let me briefly explain the tach time indication on the RPM Indicator: This is merely a geared drive from the RPM indication which in turn is engine drive via a cable. By implication, this means that the tach time is only 100% accurate at one particular engine RPM, therefore if the engine's RPM at cruise is 2300 RPM (this is where the indicator will be most accurate) and you are running the aircraft on the ground at only 1150 RPM (50% of cruise RPM) the tach time will run at half speed, i.e. indicate 30 minutes for every hour of which the engine run at this RPM. It is therefore important that the correct RPM indicator is fitted to the aircraft while on the other hand this will still imply that 100 hours of engine running time will reflect as 100 hours on the RPM tach indicator.
Now for the Hobbs indicator:
There are a number of different ways of connecting up a Hobbs meter, all of which measure a "different" time. The first and most seldom used method is connecting the Hobbs meter directly to the aircraft battery master switch which means that the Hobbs meter starts clocking time just as soon as the battery is switched on, all be it that the engine is not running. Another method of activating the Hobbs meter is via an airspeed switch or a switch on the undercarriage, which essentially measures flight time and does not record any engine running time on the ground. The third method of activating a Hobbs meter is via an oil pressure switch. As soon as the aircraft engine is started and the oil pressure exceeds a predetermined pressure the switch makes and the Hobbs meter runs.
In light of the above it can be seen that there is no clearly defined answer to your question but I would suggest that you ask your maintenance AMO to determine the method employed to drive the Hobbs meter and then you will know what time is being measured.