Zwift under reporting power

Ive been monitoring what zwift reports as average power after a session with my Garmin head unit. Im using assioma power meter pedals. In a tdz stage my garmin head unit reported ave of 283 watts… Zwift reported just 260 watts ave. Zwift said its a known bug, and to switch from ant+ to Bluetooth. It made no difference. Is anyone else reporting this?

On what trainer?

Most of my dual recordings are within 2%

Which device do you have paired as power/cadence? Is it the same as that is paired with the Garmin?

As far as I understand it, Zwift will transmit the power sent to it by the device it is paired with. It is unlikely to misrepresent this information.

My assioma pedals also transmit cadence. On the one hand zwift said its a known issue, then I get another email saying its because your using a magnetic sensor on a wheel!!! Someone in support doesn’t know the difference, especially when I pointed out I’m using assioma power meter pedals. Sometimes zwift under reports by 10 %, other times less, but never consistently… Which is the main problem… Using zwift for fun would be ok, but training and racing is another ball game if the reported wattage is not accurate or consistent :confused:

I’m Zwift, work with me :slight_smile:

What do you have paired? Assiomas or your trainer? If the latter, what trainer?

Assioma pedals via ant+, then Bluetooth to a win 10 laptop. Tried both, as per zwift support suggestion but always being under reported by zwift.

Here’s what zwift support said…

I understand you are concerned about Zwift consistently reading very low compared to Garmin’s unit Zwift averaged power at 271w and Garmin’s unit averaged 288w, right? I can imagine how concerning this might result for you, we also appreciate your feedback and the image provided, you did great! let me do my best thankfully I have a workaround to help you in one way or another today!

At this time, we’re aware of this known issue.

We are handling this situation with these steps:

  • Disconnect and reconnect Controllable on the Pairing Screen
  • If possible, switch from ANT+ to Bluetooth pairing
  • If possible, run a spindown calibration​ in Zwift

So zwift, please dont u turn and say its not a known issue, when you recognised it is…

what is paired as power source for zwift and what is connected to your garmin?

Pedal power meter should read higher than turbo power, the readings come before drivetrain losses.

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The pedals broadcast to any ant+ device listening, ant+ usb receiver, garmin cycle computer. Wahoo head unit, any compatible ant+ device. They also broadcast over Bluetooth, again to any Bluetooth receiver. Zwift is receiving the exact same data as any other device. My guess is zwift is running some algorithms that manipulate the data and spitting out the wrong numbers…

so just to be clear - your pedals are connect to the watch and to zwift?

what trainer do you use? this isn’t paired as power source in zwift is it?

Pedals are connected to my garmin cycle computer, no other devices running a power meter… There is no reason zwift should be under reporting power when everything is reading the same data from my pedals, hence my suspicions zwift is massaging the data at there end. They themselves acknowledge this is is a known issue, so we can assume others who have compared data have noted discrepancies in data reporting…

Is your Garmin head unit set to include zeros for average power? If that is turned off it could explain the higher number as it won’t include any coasting in the average power calculation.


does your garmin have auto pause on?

No, and no…

Dual record your ride and compare the files using

Found this (Trainerroad):

The interesting part is

Bryce from TrainerRoad Team advices to use Ant+ The reasoning behind this is that when you pair your dual-sided power meter with TrainerRoad via ANT+, it measures and communicates L+R power. When paired via Bluetooth, it takes the Left power and doubles it to determine your power output.

Could this be the reason?

Please include 0 Power to your Garmin Head Unit…that will make you average the same :smiley:

Power averaging include 0W
Cadence averaging do not include 0rpm.

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