Likely stuffed gauges, they were never that accurate and now nearly 40 years old. Plenty of used out there. The tacho rarely sits at 0, if off then the needle weight pulls it down, if running, well...and as you know it's only a general guide to what the engine did a few sec earlier. Unplug your oil sender and see if gauge moves, could be either. When it stops getting a signal it just sits where it was. You can also ground the sender wire and see if it moves on the gauge, which will show a faulty sender.
