Here is a great article by Sheldon Brown showing how bicycle cyclometers work. When bladed spokes came out, some units would register twice the distance at slow speed up to 6 mph 10 kph, so it can give you an idea of how fast it can register the magnetic field closing and opening a reed switch in the pickup.

https://www.sheldonbrown.com/cyclecomputer-magnet.html


Here is a bicycle handlebar bubble inclineometer if you are still interested.
https://www.amazon.com/Sun-Company-C.../dp/B06XCMXRVP