After a break after my marathon, I started riding on Zwift and got really hooked on it.
So for the past 3months, I’ve been riding with my very old “dumb” wheel-on trainer with speed and cadence sensors.
Each ride, I did a calibration of the trainer and the power on Zwift was more or less the same as what my Tacx showed on its screen.
This week I switched to the Favero Assioma Duo and all of sudden my power dropped by 20% or even more…
I’ve seen posts about Zwift overestimating power based on speed/cadence, however, now it also seems that my old Tacx Flow is showing an overestimated power.
Would this be normal?
The Assiomas have a very good reputation, and I have seen a number of YouTube tests where they are compared to many other power metering devices.
I would say the new power is very likely the true power.
Check out GP Lama…
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I Favero Assioma we trust!
Ok, so I did another ride and my initial 20% was not even close. For every power that I could see on my Tacx display, the Favero power output was 40% less… 
That being said, I do have a very old Tacx Flow (not entirely supported by Zwift), but I would have expected the power numbers to have been closer to each other then they are now.
Not sure if I should look at distance, but comparing data from my garmin and polar speed sensor, they more or less had the same distance as the ride on Zwift based on power.
So at this point I’m not entirely sure about the differences.
Hopefully this will be cleared out when doing my FTP test this weekend, so I can compare the 20min power output with expected speed.
Distance will be different. Zwift use power to calculate speed the gamin use wheel rotation.
Use the Power pedals at your primary power source. The speed sensor did it’s job.
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It seems I did not check my tyre pressure.
Did an ftp test yesterday with properly inflated tyres and power was only 10-15Watts difference.