If you are using the new Classic Instruments speedometers with ZST technology and have a speed signal from a computer, you may need to use an inline signal filter. The ZST speedometer is made to read many signals, some of which are low voltage at slow speeds. The speedometer needs to be very sensitive to read the low voltage signals and is therefore susceptible to interference that may be on the signal wire. Signals from computers are known to have interference and may cause the speedometer to “act” like a tachometer when not moving. The SN79 signal filter will eliminate interference on the signal wire and still allow the actual speed signal to pass through.