# Zwift shows half speed

**URL:** <https://forums.zwift.com/t/zwift-shows-half-speed/553654>\
**Category:** Bugs and Support\
**Created:** [July 24, 2021, 6:35pm UTC](https://forums.zwift.com/t/zwift-shows-half-speed/553654 "2021-07-24T18:35:41Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![Mike\_Rowe1](https://sea2.discourse-cdn.com/gozwift/user_avatar/forums.zwift.com/mike_rowe1/32/530428_2.png) [@Mike\_Rowe1](https://forums.zwift.com/u/Mike_Rowe1)\
**Post date:** [July 24, 2021, 6:45pm UTC](https://forums.zwift.com/t/zwift-shows-half-speed/553654/2 "2021-07-24T18:45:05Z")

</div>

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.

> [@How Does Zwift Determine My Speed?](https://forums.zwift.com/t/how-does-zwift-determine-my-speed/427729):
>
> How Does Zwift Determine My Speed? For Wheel-on Classic Trainers: Zwift takes the wheel speed broadcast by your speed sensor and the power curve (see example below) of the trainer you select on the Pairing Screen and converts that to virtual watts, also known as “zPower”. Your in-game speed is de…

---

_[View the full topic](https://forums.zwift.com/t/zwift-shows-half-speed/553654)._