magnetic weel encoder

I am testing an idea for a weel encoder let me know what u think.
I hot glued 4 magnets to a disk of plastic and mounted the disk
on the third shaft of the gear box.
the third shaft spins less than the motor but more than the weel.
i then mounted a magnetic reed switch near the edge of the disk.
wire the reed switch to ground on one end and a digital pin on
your arduino set the pin's pull up resistor and count when the pin goes low.

Probably better to use a hall effect sensor. No issues with bouncing from the reed switch then.