I had the same problem. It is caused because the mount is facing the wrong direction and your computer is swapping speed and cadence. 5 mph would mean 63 RPM of the wheel which probably what your pedal is actually doing. The wheel is actually turning at about 3 times that rate so if you check your cadence it is probably maxing out at 180 even though that wouldn't be possible. To fix this, just twist your mount 180 degrees on your handlebars or stem and try again. That way the computer is correctly lined up with the mount.
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If the modern ones no longer have that function you could look for a used computer that fills your need. I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881
Looks like that is a wired wheel sensor. Make sure the magnet lines up correctly with the wired sensor and is close enough to it. Also check for broken wires. If both those are OK and still no speed reading then the sensor is probably borked.
