Trying to calibrate something by comparison to something that is not calibrated just muddies the water. I have two tachos, one that I have just restored and another one that read about 900 rpm difference. One is about 8% low when checked against a known speed and the other correspondingly high. Spooked me when I swapped them over.
Do you have a way to attach the cable to something and spin it at constant speed for a couple of minutes , 1400 rpm should do, and time it to record one mile,? Ideally an AC syncronous motor whose speed will be steady and known and tied to the mains frequency.
One out of a desk fan or fan motor out of an oven would do. Pillar drill with a fixed speed motor or a lathe would be ideal.
Needs to turn the opposite way to a standard drill bit.
Regards
Dave
