Inaccurate Power and Speed indication

I have a TACX Vortex smart trainer and having issues with my speed and power readings while going uphill. Even though my Cadence is extremely high and my Garmin sensor speed is above 20km/h Zwift records me at 3km/h on in-climbs. This is very frustrating as it does not matter how hard I paddle or how fast I go my max will remain between 3- 5 km/h.

Most of the other riders come past me and I am sure something must be wrong as I am unable to up my Power no matter how hard I go… PLEASE HELP!

I have used Apple and Android with no luck on both.

I’ve a similar trainer and experienced similar issues… the issue resolved when I pumped up the back tyre and re-calibrated via their app (there was a firmware update on the trainer applied too).

Not sure if that will help - but worth a try.

What’s your trainer difficulty setting in Zwift?

Your Garmin speed will very rarely (if ever) match your in-Zwift speed, and the diff between the two is exacerbated if your trainer difficulty slider is set low.

Think about it at the extreme: if you set trainer difficulty to 0, there is no added/applied resistance when you hit a hill in Zwift - so you spin really fast and your Garmin speed sensor reading is high even when going ‘uphill’, but your power is what it is and Zwift speed drops as you ascend (because virtual physics!) even though your Garmin speed and cadence doesn’t change.

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Another thing is how Zwift speed is calculated. Speed is determined by your watts, the weight you entered, virtual elevation changes (your Garmin knows nothing of this), in game bike used, in game drafting and depending on the length of the climb the speed in which you start the climb (momentum). The speed in which is showing in Zwift will hardly ever match that of your Garmin.

Make sure you are also calibrating your trainer using the Tacx app.

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Good morning @Brend_Van_Der_Merwe

Some quick observations:

You should be paring as a power source and controllable trainer.


While you are on the paring screen you should see a power number when pedaling.

The garmin and Zwift will not show the same value, the Garmin think you are on a perfectly flat road with not resistance, Zwift will use your power number from the trainer and convert that to speed.

Also check that your wheel is not slipping.


Hi all, thanks for all your suggestions.
Apologies for the late reply.

Joe_Daknis_B, your tip worked. My trainer difficulty setting in Zwift was extremely high. After re-adjusting things improved. Less wear on my tire as well!

Thank you!