I have installed new Classic Instruments gauges in my car and the speedometer operates erratically.

The sensitivity of the speedometer is affected by extraneous signals around the gauge. Electronic ignition systems are typical examples of devices that emit signals that interfere with the operation of the speedometer. The solution recommended by Classic Instruments is to use a separate power wire and a separate ground wire for the speedometer. The power wire should not be the same power source used for the rest of the gauges. The ground wire should be independently ground to the chassis at a point different from the ground point used for the rest of the gauges.

We have noticed that Autometer has also experienced this problem on some of their gauges. The problem primarily arises when the gauges are powered during the starter crank cycle. Apparently, the starter motor operation and draw on the system affects the gauge. For most wiring systems, the ignition circuit, to which the gauges are normally connected, is live in the crank cycle. The solution is to separate the speedometer power from the ignition circuit and move it to the accessory circuit. It will not be live in the crank cycle but will activate when the ignition switch is moved off the crank position to the run position.

Now, more than ever, it is important to note that shielding of your electronic accessories has to be part of your wiring plan.