I tested a Mallory and Autometer tach and found them both to be off by what I consider to be quite a bit (more than 5%) at different points from 1K to 7.5K RPM.

Since most of us dont have a frequency generator, I sketched up a simple schematic (parts that can be purchased from digikey for under $20) that can be used instead. All you really need is an accurate frequency meter. Many digital volt meters are accurate to within .5% in this range (you can do a rough check by measuring the line frequency of your house AC, it should be 60Hz).

Build the circuit powering it with the battery of your car and connect the tach green wire to the output (LM555 pin 3 thru a 100 ohm resistor). The tach wire should go to this circuit only. You will need to disconnect it from the coil or ignition box to do this measurement.

Vary the 25 turn pot until your tach reads the desired RPM and then record the frequency (Hz) that produces that tach reading.

Actual RPM = DVM frequency (Hz) * 15

This allows you to check the upper RPMs without the engine running (The key or power switch will need to be on so the tach is powered up). Dont use this circuit with a voltage that exceeds 16V. A battery charger for a 16V system would exceed 16V.

Example: A 400Hz signal should produce a 6000 RPM Tach reading on the nose. One of my tachs read 5700 RPM with a 400Hz input. The Autometer Tach was way off around idle RPM range.

Note: Use a little 9V battery instead of your car battery in case you make a mistake. (or use a fused car battery power source)