I ask you help.I must calculate low motor's speed.He go around by 20 to 177 cm minute. How i can calculate is? i try with magnetic sensor but i have bad result.Please, suggest my a code and sensor

What are you looking for exactly, RPM, FPM, FPH, MPH or KPh?
This is your RPM, "20 to 177 cm per minute"
FPM (feet per minute) = diameter of wheel * 3.1415 * RPM
FPH (feet per hour) = FPM * 60
MPH (Miles per hour) = FPH / 5280
KPH (Kilometers per hour) = MPH * 1.60934

What speed? Feet per minute, miles per hour, kilometers per hour, I gave you all the calculations.
If your asking how to actually code it, then you will need some kind of sensor. Either a distance sensor from a fixed point, or a rotorary encoder attached to one of the wheels. You can even use IR as a speed gate. Take your pick.