Typically speedometers expect an input of 1000 rev/mile (being metric, the ZF/VDO combination may be different). So the Autometer is getting around 4000 pulses per mile which is ~222 pulses per second at 100 mph. At 100 mph (146.6 fps) with a 26" dia tire (6.8' circumference), the tire turns at 146.6/6.8 = 21.6 rev/sec. With 4 magnets you get 86.4 pps at 100 mph. Apparently this works just fine as Autometer makes a sensor for just your application: https://www.amazon.com/Auto-Me...gnetic/dp/B005F7VOH6 or http://www.autometer.com/unive...ic-speed-sensor.html. Their manual shows it mounted on a U-Joint http://www.autometer.com/media/manual/2650-1634.pdf At $89 I think the pricing is outrageous!
Bicycles use a reed switch for the speedo signal. You can usually hear a "tick" as the magnet passes the sensor. Reed switches are small and really cheap and have a fast response time (it depends, but 400 Hz or higher are possible. Here's one on Amazon rated at 400 HZ. https://www.amazon.com/farhop-...kaging/dp/B01IEE4PEA They can be a bit fragile so should be epoxied into a tube or something. Reed switches have a limited life, but It can be quite long: "Lifetime depends on load conditions. If switching signal loads only, many millions, and indeed even billions of switching cycles can be achieved" http://www.pic-gmbh.com/files/reedinfo_test_en.pdf contains that and more info.
Or, you could roll your own Hall effect sensor. I don't think it would be too difficult. The robot hobby people use them a lot. http://www.robotshop.com/en/hall-effect-sensor.html
quote:Originally posted by GT4Peter:
The plan is to glue some magnets on the zf ouput shafts and take this signal for the zf.
Like on a bike .