Zwift shows half speed


I’m a newbie in Zwift app and I started testing it. I found an issue with the speed shown during a run or workout. I’m using Garmin Sensor 2 (speed and candence) with Zwift PC. I connected the sensors over my smartphone with Zwift companion.
In parallel I’m using my smartwatch Garmin fenix 6 pro which is showing correctly the speed and the distance.
You may ask me: why am I sure of my smartwatch display. In fact I measured the perimeter of the tire (700x25) and tested for each 10 meters I should have 4.7 turns of speed sensor.
So during the run when my smartwatch is showing 10km/h Zwift is showing 5km/h. I also checked metric system option in the parameters.
Knowing that cadence display is ok: smartwatch and Zwift are showing the same value.
Is there someone else who did encounter a such issue ?

Thank you

Hi @sofiene_litayem, welcome to the forums. Please read this to understand why they won’t match. Your Garmin doesn’t know what is happening in the virtual world.


Thank you for directing me to this topic.
So as understood, zwift will get the speed from the sensor as a “raw” data and converts it to a power called zPower according to the home trainer curve selected in the menu (I have a classic configuration).
In the graph: x axis is the raw speed and Y axis is the zPower in watt.
Example: if raw speed = 40km/h → zPower = 200w.
From this zPower value (200w) zwift will compute the virtual speed (the speed in the game) based on several factors: the zwifter weight, his height, the elevation changes etc … Finally, the virtual speed can be 45km/h or 12km/h according to all previous parameters … right?