As far as I know the only place to get this information is the tachometer specifications (or at least I know Mitchell provides it for their tachometers).
As you noted, it's not on the TCDS, nor have I ever seen it in the (Piper or Lycoming) maintenance manuals. (It is of course possible I've missed it in the maintenance manuals - they're pretty thick books.)
If you are flying around in a plane with a working tachometer and don't know the hour-meter's equivalence speed you can always determine it with a watch: every 6 minutes is 0.1 hours, and when the tach is recording at that rate you've found the equivalence RPM.
(For the Archer I assume it's the same as my Cherokee 180 - around 2300 RPM.)
It's worth noting that while you would think the tach's hour meter would be "accurate" (1:1 correspondence) at typical cruise powers that is often not the case!
Using the Archer as an example, 65% power cruise at 7,000 feet is around 2500 RPM, so the tach hour meter will be running slightly faster than 1:1 in a 65% power cruise at 7,000 feet.
The watch-check I described above doesn't help you if the reason you need to know this information is your tach has died and you need a replacement - fortunately tachometer manufacturers all know this and publish recommended applications for their tachometers.
Mitchell lists their
D1-112-5023 (2300RPM) tach as the appropriate replacement for the PA28 family, which matches up with the watch-test on my Cherokee.