Speed is not accurate

I am new to zwift , i noticed that my speed in not accurate at all , i am using roller trainer untitled on zwift with speed sensor attached to the rear wheel , some times it gives me 4 kph on zwift while in same time gives me 30 kph on another cycling app. , i tried several time to restart the connection but nothing changes , please tell me what to do with this issue .

Hi @Youssef_Raouf

Welcome to the forum,

You wheel speed on your cycling computer will not match you speed in zwift.

Zwift use a speed power curve to calculate the in game power . So the faster the wheel spin the more power. Zwift then use that power to calculate your speed taking your weight, height and road profile into account.

A typical power curve look like this.

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I am also going to post this:

Zwift takes the wheel speed broadcast by your speed sensor and the power curve of the trainer you select on the Pairing Screen and converts that to virtual watts. With those watts, the weight entered, height entered, in-game bike used, in-game drafting (Tri and TT bikes get no drafting boost), in-game wheel set, in-game road surface, and in-game virtual elevation changes your in-game speed is determined.