Very Inaccurate zPower Readings

Hello all,

I am receiving a very inaccurate power estimate with zPower.

My set up is a Tacx Blue Matic Classic trainer, along with the Tacx speed and cadence sensor and GARMIN HR strap connected a Macbook (using an ANT+ stick).

Last week whilst using the 7 day free trial, I had much different results than I have had over the last two days. Initially I was receiving a much higher power output than expected, but that turned out to be I hadn’t set up the trainer correctly. Once this was sorted, the power output was a lot more believable/accurate. I did an FTP test with the outcome of 339, and I could hold over 200W as a comfortable pace with my heart rate being around 60-70% MHR, which is similar to what I’ve had on WATTBIKES. Zwift was also giving me an accurate calorie burn as well, similar to what I’m getting on GARMIN.

I’m unsure if it is related, but then I set up and started paying for the Zwift subscription on Wednesday 24 March. The first time I’ve used the bike since was this weekend. The bike set up has not changed or been touched at all and I’ve been getting much different results. Because of this, I did another FTP test today with a result 133, I could barely get into it as my power was so low. With the same amount of effort I’m getting a much lower power reading, where I would have been expecting to be around the 200W range I am now getting about 60W. Also although I am still getting an accurate HR reading on Zwift my calories burnt has reduced as well. On a workout today I burnt over 600 cal according to GARMIN, which is what I would have expected, but Zwift told me a had burnt just over 200 cals (after having an avg HR of 131 for 50+mins). I assume this is connected somehow as well, as it seems to think I am just not putting the effort in, my HR and towel say different.

I’ve checked sensor batteries, tyre pressure, roller tension, connecting with Bluetooth rather than ANT+, uninstalling/installing, my weight and MHR on my profile, all of which are correct. So I am confused to what has changed.

It is frustrating seeing such inaccurate results as soon as I have started paying for the service, the values are so low I don’t even think I could complete a race currently without it taking a lifetime.

If anyone knows any suggestions to what could be causing this I would be greatly appreciative. I honestly don’t think it is the set up as nothing has changed. It is almost like the power curve for the Tacx Blue Matic has been massively reduced, if this is what is causing it, do you think it will get rectified soon?

Thank you. Looking forward to a response or a fix.


Have you updated Zwift during this process? Because there was a bug introduced in the March 16 release that caused trainers like yours to report way too much power, and that bug was fixed in the March 23 patch, bringing the power back down again.

Cheers for the response Steve. The App has automatically updated. If it is this, it has seemed to cause more of a problem than its fixed.

Have you checked your setup? You say that you haven’t touched it, but tyres can easily lose pressure, and having the correct pressure and roller tension is critical to getting vaguely accurate power figures.

I had checked the tyres and tension. The only difference it made was it pushed the low power towards the higher gears, and unfortunately not the other way. I’ve email customer services now and they’ve asked for the ride logs and escalated it further. I am just waiting for a further response.