Used zeroed speedo in kilometers

[Nico911]

Hi guys.

I'm wondering what is the most common/easiest way to get a speedometer (in metric in my case) from the '80s and have it zeroed properly ?


Thanks in advance.

Nic

[Dave Colangelo]

If you have a friend with a wave form generator he may be able to help you. I do know from playing around with my speedo (78 SC) that they take a square wave 12V P-P between 0 and about 300Hz. I dont know if the Carrera speedo's took the same signal but they should take a square wave of similar type, if being driven from the ECU it may be a 5V signal. I dont know exactly what frequencies match what speeds but if you have a friend in the area with a known good speedo that will help. Other wise you will have to get an oscilloscope hooked up to your car while its moving and have some kind of a speed reference to read while you measure pule width. How far off is it? Have you compared it to say a GPS speed. If you have remember they will have different readings. Most GPS units calculate speed over a short distance, say 100 meters or so, your speedo is near instantaneous speed of the vehicle, although close you may see some discrepancy in these numbers.

Or I'm sure one of the place that rebuilds gauges can take care of this for you.

Regards
Dave