if you want exact engine speed, you need a calibrated tach. the calibration costs more than the tach itself. or simply compare a good shop tach with the dash gauge and adjust the dash gauge as reqquired.I prefer a non-contact photo tach to get me the baseline RPM and I pull the back of the tach apart to get to the trim pot on the circuit board to dial it in if needed.
most gauges in boats cost the boat manufacturer about $5 each for a tach and speedo and a bit less for the other gauges. they are all merely a suggestion as to what is going on. The exception is CAN networked gauges such as Smartcraft technology, etc. those are about $25-50 ea and a bit more accurate as they are taking CAN data and displaying data messages thru a small processor
for stand-alone gauges - analog gauges are no more or less accurate than digital gauges. they both take a pulsed digiial signal thru a bit of simple electronics to convert it to a display. the analog gauges simply convert to voltage and the air core motor moves the pointer. inexpensive digital tachs like the OMC Quartach unit you mentioned are inexpensive digital to analog to digital systems. they are slow to respond and still only a suggestion of what is going on.
however if it makes feel better to see a numerical display vs a sweeping gauge, then by all means, get one. they are no more or less accurate than any other properly setup commonly available marine tachometer
