@darkshvein, a common issue at this maxis from Kymco.
Pls try:
- clean the speed sensor and rotor. take it off your wheel (sensor only)
-use alcohol or fine fuel for it
-mount it again with a distance 0.8 mm between sensor and this "rotor"
-have a test ride
-improved Y/N
-if NO ,change the distance with thin (alu foil) shims either to wider or more narrow
-Y/N
-take a multimeter and measure resistance
IF after several different "distances" your speedo does not get more accuracy, forget further efforts and live with it. An electronic smurf causes that in our scoots.