??? you must be an mechanical engineering guy as those had a rate of +60% to fail an level 1 physics examination at my university.

perimeter multiplied with your magnets count gives you distance or
diameter*Pi*counts=distance relative to time you get d*Pi*x/t=distance/time=speed with a nice program you could track acceleration too

that's basically how bulk bike computers work, you input your wheels diameter and install the hardware. and they are as precise as you measured your diameter (if you didn't just read of the tube)...

btw gps isn't that good for low speeds as the precision is most of the time within a >3m radius, the faster you go the smaller is your "measurement fault" relative to "distance traveled"
I know that you get movement recognized within the radius but moving 7m/s (bike) or doing a 20m/s (car) makes a difference
measuring or compute (for gps) your acceleration in real time needs a bit more precision then gps.

and now where I wanted to go with it!

car developers use cameras, as you be able to measure acceleration in all directions at the same time, I know that sounds awesome to program and you need to measure the exact position of the camera for precision but it's still an incredible idea, isn't it?