The part that caught me off guard first: the tachometer sitting in this 1973 Nova started life as a temperature gauge in a 1998 Jeep Cherokee instrument cluster. Turning a coolant gauge into an engine speed readout is the kind of parts-bin creativity I always enjoy reading about.
The way it came together is tidy. The temperature gauge was cut out of the donor cluster and its needle coil got a second career showing RPM. A custom circuit, built on perfboard with components already on hand, conditions the ignition pulses and filters the 400v spikes that a points ignition throws out. It also adjusts the ratio of incoming pulses so the needle positions line up with actual RPM, across a scale running from 0 to 8000. The whole thing fills the blank clock slot on the right of the dash, leaving the stock cluster untouched.
That slot choice matters. The factory tach option took the fuel gauge spot instead, and no suitable aftermarket unit existed, since the smallest generic one found was 52mm against the 42mm needed. An old aftermarket alternative went for 300 dollars back in 2010, so a scratch-built answer makes sense to me.
It is not done yet. The current face is temporary white printer paper, the sunfaded needle is due for an orange repaint, and the schematic has not appeared so far, though the maker plans to share it Friday night. One open question is whether the circuit would drive gauges from other cars without swapping two or three resistors.