The wired ones can be affected by ignition too. I tried screening the cable from pickup to readout on one inexpensive unit, which helped a bit but it would still fail. IIRC the reading intermittently locked up & required the battery removing to get it working again.

I have a Sigma Sport 8 unit on my MZ (mainly to get a clock & trip meter) that was fitted to an RE Classic 500 (4 speed) when I bought it. That seems to have been designed for vehicle use, but doesn't seem to be available now.

I set the scaling factor by running a piece of thin cable around the center of the tyre & cutting where it ovelapped to get an accurate circumference. The Sigma is natively Metric, so the measured circumference has to be divided by 1.61 when set up for Imperial - when set like this the MZ speedo very accurately tracks the digital one.

When I originally started riding the RE I used the Sigma speeedo as the RE unit seemed out by a long way. Subsequently found that the previous owner had not set the Sigma up correctly - when the correct scale factor was set the RE speedo also turned out to be accurate & by using the Sigma incorrectly set I had been running well over 10% faster than the speed limits.

Nigel B