The Toyota tachometer accepts a pulsed ignition signal from the coil. This signal is converted into a proportional DC signal that drives an analog current meter in the dash to display engine rpm. Its reading can be scaled to correctly display the correct engine rpm by adding a calibrated resistive shunt to the meter input.

A 5k ohm trim potentiometer is placed across the input contacts to the tachometer meter. This is done directly at the meter posts inside the instrument cluster. For this purpose, it is best to use a small 10 turn potentiometer to provide easier calibration of the tachometer.

This potentiometer can be purchased at almost any electronics store. Solder wires to the potentiometer and find a way to add it to the wiring at the back of the meter.

The figure here shows schematically how this is done. Once installed you will be able to recalibrate, or scale, the tachometer reading by adjusting the potentiometer. By running the engine and using another accurate tachometer as a reference, adjust the potentiometer until the engine rpm readings are both the same. Once done, you will have a factory tachometer that reads accurately for your V6 or V8 engine.