When you buy a bike computer that measures speed and total distance moved, there are three parts: the computer itself with the little screen that you attach to the hadlebars, a magnet that you attach to the spokes of your front tire, and a magnetic sensor that you attach to the fork of the bike in line with the rotating magnet in the spokes.
When you set up the computer, the instructions say that the only measurement you need is the size of your wheel, and that the placement of the magnet and sensor in the spokes can be at any place between the center of the wheel and the edge. Why does it not matter where you put the sensor when calculating the speed and total distance?