Wheel encoder

1 screw with diameter 10mm (17) and length 80mm (I used 100mm length on pictures)
4 nuts and 2 washers
1 wheel diameter 75mm
1 optical endstop
1 or 2 springs
1 hinge

you can with a GPIO on a raspberry pi count the number of tooth and convert in speed.

Example:
On my print I have 2 tooths so if in one second I count : 1 - 0 - 1 - 0 - 1 - 0
3 tooths was found so : 1.5 round
nb tooth counted / nb tooth by round x whell diameter x pi
3 / 2 x 75 x 3.14 = 353.4 (mm) so ~35cm/s