Tachometer installation

I would set it to 4 (assuming you have a V8) and compare to another tach.

if the car is a manual you can do some math or find a calculator, (Spicer Engine RPM Calculator) plug in your tire size, rear gear ratio and go drive at a speed in top gear ( 3rd if 3 speed 4th if 4 speed, no overdrive) at a particular speed your tire/gear combination will force the engine to turn over at a particular RPM (like 2000 RPM to go 60 MPH) if the tach shows about that (a couple hundred up or down) you have the correct setting.

If the car is automatic trans there is some loss with the torque converter so the RPM would be a tad higher than for a manual.